Luna discovers that her grandmother's magic belt is now in the possession of Mumm-Rana, who claimed it after battle. She heads straight for the white pyramid, with Pumyra hot on her tail.

The story of Excalibur, the greatest sword in history, is revealed to Mumm-Ra, who is then determined to find the Excalibur and defeat Lion-O and the Sword of Omens.

Mumm-ra transforms himself into a ro-bear berbil (a Thundercat ally). He acts hurt and the Thundercats let him stay at Cats Lair. One night, Mumm-ra(the berbil) puts all the Thundercats under his spell, except Snarf, because Tygra held off Mumm-ra just long enough for Snarf to escape. Lion-O was sleeping out in the woods, and Snarf ran to him and told him what happened. Lion-O came back and fought Mumm-ra, who was actually beating Lion-O. Mumm-ra was wearing special glasses so he couldn't see his reflection. Eventually, Lion-O used the Thundercat signal, broke Mumm-ra's spell and his glasses, and finally defeated him with his reflection.

Wily Kit and Kat stand in Lion-O's way on the third day of his trials. Lion-O must beat them through the maze of infinity. Attempting to stop Lion-O and slow him down, the kids set traps in the maze. He manages however to avoid their traps. In order to beat them through the maze, Lion-O plays their own tricks on them, and tricks them into losing the race through the maze.

By disguising himself as Jaga, Mumm-Ra tricks Lion-O into believing the new ThunderCats are traitors so that they will fight amongst themselves.

An evil robot monster is attacking the Tuska Village, in an area unexplored by the Thundercats. Turmagar, the leader of the Tuskas, travels to the Cat's Lair to ask for aid in defeating the Techno-Pede robot which is destroying his village. The Thundercats agree to help Turmagar, and set out to the Tuska Village.

While trying to figure out a way to read the Book of Omens, Lion-O tries looking through the sword at the book. He is immediately drawn into the book, where a voice asks him if he has ""the key."" Lion-O says he doesn't, and four dragon heads appear and shoot fire at him. Snarf, who saw Lion-O get sucked into the book, tries to show Cheetara what happened, and he too gets sucked in. Mumm-Ra, who also watched Lion-O get sucked in, goes to cats lair and takes the book so he can destroy it with Lion-O inside. Meanwhile, just by coincidence, Panthro finds an odd looking key on Thundera. He and Tygra head back to Third Earth with the key, not knowing what it's for. Back on Third Earth, Mumm-Ra is just about to boil the book when Lion-O calls for the sword, and it brings him and Snarf back outside. Mumm-Ra takes the sword and looks through it at the book, and he gets sucked in! But Lion-O has to save him because of the code of Thundera, and Lion-O does save him. Panthro and Tygra arrive with t

The normally submissive JackalMan decides to start his own army in hopes of conquering Third Earth by himself. He does manage to give the Thundercats trouble, but soon he is defeated and ends up crawling back to the other mutants at Castle Plundarr, who weren't very happy that he rebelled in the first place.

The Thundercats use a new Tower signal to guide a Thunderian ship in; aboard are Baron Tass and Mr. Grubber, a devious pair of Thunderians. Later Lion-O sends Torr, Baron Tass and Mr. Grubber to get water, but on the way the Baron stops at the Well of Doubt and gets some of its water. The water causes a person to become very doubtful of his own abilities. When they get back to the Thundercats, Baron Tass gives some of the water to Lion-O. On the way to inspecting the new Tower in the Whisker, Lion-O feels the effects of the water and causes the Whisker to crash. Tass comes back (from getting his Guardsman, giant stone-men), and has them attack the Thundercats. Jagara has the others hold the Sword, and when Lion-O grabs hold of the sword, he's free of the water's effect. Then he fights off the Guardsman, and makes a storm, which causes the Guardsman to fall apart, and Tass promises to change his ways.

Vultureman escapes from exile by hijacking a bookmobile. He finds a disk containing ancient history about two powerful evil forces on Third Earth: Mumm-Ra and his enemy Zaxx. Zaxx used a Rosencrest medallion, but Zaxx was defeated when Mumm-Ra stole the medalion trapping Zaxx in it. Vultureman plots to get the medalion, defeat Mumm-Ra, and rule the universe. So Zaxx uses Vultureman to find and defeat Mumm-Ra. But there's a terrible ""side effect"" to the medalion, anyone who wears it loses his own life and BECOMES Zaxx. When Vultureman learns of this, he changes his mind. The Thundercats get caught in this situation when Vultureman attacks Panthro and Snarf and after they crash, Snarf dissappears. Zaxx then uses Snarf to attack Mumm-Ra and Zaxx takes over Snarf's body. The other Thundercats arrive and save Snarf. Mumm-Ra defeats Zaxx once more by takin the medalion, but the pyramid also collapses on Mumm-Ra.

Mumm-Ra orders Amortus, a being he banished to the Land of No Return centuries ago, to destroy Lynx-O. Amortus makes Lynx-O crash in the Thunderstrike, and then he touches Lynx-O's face. The spell from the touch makes Lynx-O feel like he is an outcast, so he leaves to join Amortus in the Land of No Return. The other TCats, Lion-O, Panthro, and Cheetara, go after him. In the Land of No Return, Amortus again touches Lynx-O, this time causing him to turn to stone. The other TCats arrive and try to beat Amortus, but he touches all of them, causing them to start turning to stone. But just as Lion-O is about to turn entirely to stone, he calls the Sword and it breaks them free. It also releases other warriors trapped by Amortus, and they all come and defeat him. Lynx-O is rescued just in time- just before the stone is permanent- and they all go home

Mumm-Ra summons the Mutants and threatens to banish them from Third Earth unless they bring him the Sword of Omens.

When a big storm hits Third Earth, the Thunderkittens are trapped in an old space capsule. Meanwhile, Tygra and Cheetra almost crash the Feliner, but Lion-O guides them in for a safe landing with the eye of Thundera. The kittens are captured by Mumm-ra, but end up saving the day by showing him his reflection in the window of the capsule.

Mumm-Ra tells Alluro where to find a talisman that boosts the self-confidence of the one who possesses it so that they become nearly invincible. Snarf learns that true courage comes from within.

Willa is practicing her long-bow skills when a warning device alerts her to a Mutant attack. Willa is captured and the ThunderCats rush to rescue her.

The Mutants, who have also reached Third Earth, encounter an old pyramid. It turns out to be the home of Mumm-Ra the Ever-Living, and they join forces to capture the Sword of Omens. Both the Mutants and Mumm-Ra attempt to attack Lion-O to possess the Sword, but miserably fail.

From the Shadow Realm, the Shadowmaster contacts the Ancient Spirits of Evil (ASOE), telling them of how he captured Lion-O's father, Claudus, just before Thundera blew up. Then the Shadowmaster, through a bad dream, shows Lion-O his captured father and challenges Lion-O to a fight. Lion-O goes to the Shadow Realm through the Book and Sword of Omens, and is able to fight the Shadowmaster as long as the Sword stays bonded to the book. However, the Shadowmaster sends a huge bat to steal the Sword. It succeeds, and then Lion-O becomes very weak in the Shadow Realm. Mumm-Ra meanwhile, feeling like the ASOE will turn against him in favor of the Shadowmaster, sends Ma-Mutt to secretly steal the sword from the bat and put it back near the book. This renews Lion-O's strength, who calls for the Sword, rescues Claudus, and exiles the Shadowmaster. Claudus leaves Lion-O, but at the end you see him and Jaga standing next to each other in that blue, ghostly form. Now Lion-O has 2 mentors to give hi

Lion-O undergoes the fourth task in his Anointment Trial.

Vultureman builds a sonic gun from a sound stone he stole in Dark Side as the Lunatacs join with him to attack the Tower of Omens.

Mumm-Ra creates a museum where painting and statues come alive to oppose the ThunderCats.

To prove themselves worthy of being ThunderCats, the ThunderKittens borrow the other ThunderCats' weapons and practice with them. They are ambushed by Mutants, who take their weapons.

With the Mutants and Lunataks gone, the Ancient Spirits of Evil give Mumm-Ra until sunset to destroy all the Thundercats, and to prove they're not bluffing, they exile Ma-Mutt. Mumm-Ra spreads destruction all over Third Earth, sends Tygra, Cheetara, and Panthro to limbo, and buries Lion-O alive. Then he attacks and destroys the Tower of Omens, and the Thunderkittens barely escape, but Mumm-Ra sends them to limbo too. Snarf must go into the Book of Omens to find out how to defeat Mumm-Ra. Jaga and the Thundercats in limbo go into the Eye of Thundera to help Lion-O free himself from his rocky prison. Lion-O then battles Mumm-Ra at Cats Lair and finds out from Snarf that Mumm-Ra only has until sunset to defeat the Thundercats. Mumm-Ra fails and the Ancient Spirits of Evil banish and exile Mumm-Ra from Third Earth. With all their enemies gone, the Thundercats decide to permanently return to New Thundera

A bounty hunter known as Captain Bragg comes to Third Earth to help the Thundercats get rid of the Mutants and the Lunatacks. Capt Bragg tricks Wilykat in going with him and they first go after the Mutants. After capturing them, they go to capture the Lunataks. They capture all except Luna and Amuk, who free the other Lunataks, put Bragg in a cage and tie Wilykat to the train's engine. Lion-O and Wilykit arrive to save Wilykat and the Lunataks are defeated. All the Mutants and all the Lunataks are captured and exiled.

The concluding trial in Lion-O's Anointment Trials pits him alone and weaponless against the evil Mumm-Ra.

Mumm-Ra uses his shapeshifting abilities to pass himself off as a Thunderian so he can stage the "rescue" of Cheetara and infiltrate Cats' Lair.

Panthro, Tygra, and Cheetara return to New Thundera in the Feliner to find more of the Treasure of Thundera, but Mumm-Ra shoots the Feliner down and, as the Fog of Despair, sends them to the Canyons of Youth, which changes them back into Thundercubs. Then Mumm-Ra disguises himself as Lion-O and tells the Thundercubs to find the treasure for him. Meanwhile, back on Third Earth, Lion-O and Snarf decide to go rescue the Thundercats on New Thundera, but they must ask the Mutants to take them to New Thundera in the Rat-Star. The Mutants agree, secretly planning to attack Lion-O once they're in space. They do attack and imprison him and Snarf on the way to Thundera, but they manage to break out. Cheetara meanwhile, using her sixth sense, locates part of the treasure, the Mirror of Truth. The real Lion-O shows up, and uses the Sword of Omens with the Mirror of Truth to restore the cubs to their proper age, and Mumm-Ra reveals himself and turns into Mumm-Ra the everliving. Cheetara uses the mi

New Thundera is being torn apart and the Thundercats must present certain artifacts at various dangerous locations that used to be beautiful places, then Lion-O must go into the book and give the Key of Thundera to the guardian of the book ""at the 24th hour,"" or the Thundercats will be frozen in time and New Thundera will be destroyed. But Mumm-Ra steals the book and key and journeys into the book himself. He wants to save New Thundera BUT wants the Thundercats frozen in time. The Ancient Spirits of Evil try to stop Mumm-Ra because they fear if Mumm-Ra goes into the book, their powers will cease to exist. The other Thundercats run into problems also at the locations they are supposed to be at. Lion-O goes into the book using the Sword of Omens and battles Mumm-Ra one on one, neither of them using any magic. Even when Mumm-Ra cheats, Lion-O still wins. The ASOE summon their ""champion,"" Pyron, to then fight Lion-O, but Lion-O gets the dragons in the book to fry him... He presents the key

Jackalman ambushes Snarf and attempts to steal the Sword of Omens. This gives Panthro the idea to make a duplicate sword as bait for the Mutants.

On Lion-O's 20th birthday, he must pass a set of trials to become the true Lord of the ThunderCats. Even though he is the hereditary Lord of the ThunderCats, he must prove himself to be worthy of that title. His first test is to beat Panthro in a duel to test his strength.

An Ecology Inspector (EI) comes to Thundera to inspect the new Cats Lair. His ship uses the SWAN system (Space Way Airborn Navigations) to direct him to planets all over the universe. When he arrives on Thundera, he gets ambushed by Two-Time (see Return to Thundera, Part III). Two-Time uses a hologram to make the EI's ship crash into a mountain, and then when the Thundercats come to rescue the EI in the Feliner, Two-Time uses the hologram again. Lion-O manages to fly around the mountain and lands the Feliner. Meanwhile, Two-Time flies to new Cats Lair in his spaceship, Dome-Down. He actually picks up the whole Lair and keeps it inside Dome-Down. The Thundercats fix the Feliner, rescue the EI, and then fly to catch Dome-Down. Once they catch the spaceship, Lion-O uses the sword to defeat Two-Time and send him flying out of the ship. Then they return Cats Lair to its proper place.

After another earthquake caused by Mumm-Ra ruins the new Cats Lair construction site, the Thundercats go to see Jaguara and confront Mumm-Ra. Unfortunately, Mumm-Ra puts up a force field over entrance. Jaga appears to Lion-O and tells him to use the Eye of Thundera to get to Jaguara and the Gyroscope. Lion-O does so and battles Mumm-Ra, who throws him into the bottomless pit underneath the Gyroscope. Lion-O however, manages to throw his grappling hook to the top, which pulls him back up. Lion-O then shines the signal, which goes through the force field. All the Thundercats leap through the signal which takes them through the force field. Together, they fire all their weapons at Mumm-Ra and he is defeated. But before he leaves, he breaks the cord that holds the Gyroscope above the pit and it falls through. Now, New Thundera is literally falling apart. But Lion-O throws the sword in the spot where the Gyroscope used to be and calls upon the Thundercat Ancestors for help. The sword starts

While Hachiman is on his way to the Tower of Omens, Luna steals his sword, the ThunderCutter, and gives it to an automaton. Lion-O and the ThunderKittens head out to find their old friend.

The ThunderCats look for Sky Tomb to free the slaves imprisoned inside it. Bengali must learn the importance of teamwork as Lion-O leaves him with guard duty. Lynx-O and Cheetara run reconnaissance to find out if Mumm-Ra is truly back or not.

Mumm-Ra sends Ma-Mutt disguised as Mandora to free the Mutants from Capt. Bragg's circus train and sends Bragg and Crowman falling into deep space. The Mutants are then ordered by Mumm-Ra to get rid of Tygra and Pumyra. The Mutants attack Snowman on Hook Mt. and Snowmeyow gets Pumyra for help and Tygra contacts the real Mandora to save Bragg and Crowman. Then together, they recapture the Mutants. Meanwhile on New Thundera, the earthquake tremors grow worse and the new Cats Lair collapses trapping Snarf in the ruins. Lion-O uses the sword to contact the other Thundercats and they rush to the Lair to save Snarf. Afterwards, Mumm-Ra destroys the Gyroscope's regulator...

Mumm-Ra is weak and faces the threat of three new ThunderCats; he sends the Mutants to release his old enemies, the Lunatacs, imprisoned in Dark Side[29] in order to bolster the forces of evil. The ThunderCats' celebration of their victory over Mumm-Ra and the welcoming of three new ThunderCats is cut short when Snarf overhears plans at Castle Plun-darr after he chased Ma-Mutt there from the Berbil village.

Jaguara cannot work New Thundera’s Gyro on her own. The Thundercats order a robot to aid her in running the Gyro. Screw Loose, a robot who specializes in Gyro’s, comes to New Thundera. Mumm-Ra manages to mess up Screw Looses brain so he becomes destructive instead of helpful. The Thundercats must help Jagara get Screw Loose under control, before the entire planet is destroyed.

With their home planet, Thundera, about to explode, the Thunderians flee to a distant galaxy. Before they can make the long journey the Mutants sweep in and destroy most of the fleet in an attempt to capture the Eye of Thundera. Due to the attack, the Thundercats find themselves on a distant planet. A grown-up Lion-O must save his Thundercats from the Mutants on Third Earth.

Wily Kit and Kat are constructing a well for the New Cat's Lair on New Thundera. The Thundercats on New Thundera receive a distress signal from Thunderian refugees trying to find their way to New Thundera. The Kittens are left alone with Snarf on at the New Cat's Lair, and Lion-O and Snarfer are out constructing an early warning device (I guess for weather purposes). The rest of the Thundercats seek out the refugees in space. When Wily Kit and Kat strike water, they accidentally unleash a monster that Jaga trapped in the underground lake centuries ago. Frogman, with aquatic powers, tries to sink the New Cats Lair. Lion-O however re-traps Frogman in his watery prison.

A Terator Starship, an extremely ugly spaceship, is pursued by a Kymera Craft, a visually appealing spaceship. The Terator Spaceship escapes and lands at the Cats' Lair

Mumm-Ra has a phial of miniaturization gas. Vultureman builds a mechanical Wolfrat to infiltrate the gas into the Cats' Lair. The ThunderCats are immediately reduced to nine inches high. Knowing how vulnerable the ThunderCats are in this state, Mumm-Ra orchestrates a mutant attack on the Cats' Lair. The ThunderCats' only hope is Snarf; they armor Snarf and use him as a battle charger while Tygra works on an antidote. The ThunderCats finally take the antidote, and Snarf, getting an accidental whiff of it, grows to huge proportions. In his gigantic state, Snarf terrifies the mutants and puts them to flight.

While all the ThunderCats except Lion-O are gone, Mumm-Ra switches Ma-Mutt with Snarf. Ma-Mutt poisons Lion-O, allowing Mumm-Ra to move in.

Mumm-ra finds the Totem of Dera (a part of the T-cat treasure) which has special healing powers and can bring inanimate objects to life. He gives it to Vulture-Man and the Mutants so they can make a device to destroy the T-cats. After making the device, the mutants decide to test it near a swamp. At Cat's Lair, Lion-O sees that there is some mutant activity near the swamp. He, Wilykit, and Snarfer decide to check it out. When going to the swamp, the Hovercat is brought down by a swamp monster made by the device. The Hovercat crashes in the swamp, and Lion-O gets intoxicated by the deadly swamp water. Jaga appears and tells Snarfer that he must get the Totem of Dera to heal Lion-O. Snarfer gets the Totem and succeeds in healing Lion-O. Lion-O calls on the T-cats to try to kill the swamp monster. When all their weapons prove ineffective against the monster, the T-Cats come up with the idea to dissolve the monster in water and they succeed in defeating it.

Nearly all the Thundercats have been captured by the Mutants and imprisoned in Mumm-Ra's Pyramid, forcing Snarf to use his own talents to free them.

Wilykit and Kat accidentally free an ancient monster that was banished to an old shrine centuries ago. The evil monster, Mongor, begins to destroy the planet, and is much stronger than the Thundercats. He captures all the elder Thundercats with his powers. Wilykit and Kat return to the shrine to find a way to destroy him. They report back to Lion-O and tell him that in order to destroy Mongor, he has to fight the monster without looking at it. Lion-O defeats the monster and all is returned to normal.

When a meteorite made of pure Thundrillium crashes on Hook Mountain, the Thundercats decide to use it to replenish their depleted energy supply. But Lion-O must first gain the trust of the fierce Snowman.

The Thundercats capture the Lunataks and, tired of dealing with their evil ways, decide to exile the Lunataks to Exile Isle, an island in space far away from Third Earth. When the Lunataks arrive, they meet up with Captain Cracker, a pirate who the Thundercats also exiled. Together, the Lunataks and Capt. Cracker send out a distress signal to a passing ship. When the ship comes near the island, the Lunataks send up a cloud of fog, which makes the ship crash. They use the engines of the ship to push the planet out of orbit and towards Third Earth. Snarfer, on his way home from Thundera, crashes on the island when it runs into him. Lion-O goes to rescue Snarfer and stop the island, because it has a deadly gas that will blow up if it enters Third Earth's atmosphere. He goes without the Sword of Omens. He gets cornered by Capt. Cracker, but the Sword awakens and flies to Lion-O, without him even calling it. He then uses dirt to stop the engines, and the planet/island eventually returns to

Mumm-Ra finds out that the Kingdom of Webs has a new Queen, Spidera. Mumm-Ra thinks he can lure Lion-O into her lair. He transforms into a diamond fairy and casts a spell on Lion-O. Snarf follows them and gets trapped in the Kingdom of Webs. Lion-O tries to save him but ends up having to call the other Thundercats. Eventually they confront and beat Spidera.

Fed up of feeling useless, Wilykat convinces his twin to run away from home, but living on their own is more dangerous than they realized. The other ThunderCats search desperately to find them as they all must learn a lesson in communication.

Vultureman builds a Thundranium[28] projector, capable of creating concentrated regions of Thundranium. When the ThunderKittens damage it, he damages their spaceboard, sending them off into danger. While Lion-O and Snarf search desperately for them, Vultureman plans his revenge.

A vagabond space traveler lands on Third Earth to challenge Mumm-Ra to a fight. The Demolisher is victorious against Mumm-Ra and sends him running back to his tomb. Mumm-Ra acknowledges the power of the Demolisher, and convinces him to seek and destroy Lion-O. The Demolisher, searching for another fight, gladly accepts Mumm-Ra's advice. He battles with Lion-O only to face defeat.

Mumm-Ra and the Mutants plot to open the Time Warp Prison and release the evil Princess Ta-She in order to use her deadly Doomgaze against the Thundercats.

Aluro invents a ""booster"" potion in hopes that it can make Sky Tomb able to fly to Thundera. It doesn't work, and he throws the potion into the forest. Then he notices everything in the forest has grown huge. He and Luna give the potion to the Mutants, who sprinkle it around Cats Lair. The Mutants also capture Lynx-O and Snarf. But while the Mutants are preparing to transfer Lynx-O and Snarf from their cell, a mouse in the cell who Snarf had befriended gets some of the potion and attacks the Mutants. That lets Lynx-O and Snarf analyze the potion and come up with an antidote. Meanwhile, Lion-O, Pumyra, and the Thunderkittens are fighting off huge bugs in the forest that has grown around Cats Lair. Lynx-O and Snarf arrive, sprinkle the antidote over the forest, and then watch as everything turns back to normal.

Micrits, a tiny race of human-like figures, happen to live right outside Cats' Lair. Unaware of their existence, Lion-O manages to continuously step on their village and destroy their homes. Upset at this disrespect, the Micrits decide to take a stand and manage to trap Lion-O. With the Thundercat Lord in trouble, the Mutants attack Cats' Lair. After much talk, Lion-O manages to convince the Micrits that he is a "good guy" and convinces them to let him go to help save his friends. Once again, Lion-O saves the day, and befriends the Micrits.

While investigating an earthquake, Tygra stumbles across the Garden of Delights and meets a talking plant named Silky, who offers him her fruit. But it turns out that "Silky" is another of Mumm-Ra's disguises and eating the fruit seems to have affected Tygra's mind.

Wilykit and Wilykat try to stop a band of living gargoyles from robbing a Wollo but fail to do so and trail the gargoyles to a nearby Tower. Wilykat is trying force the door open when he suddenly disappears, leaving Wilykit - who was looking for another way in - close to panic. She and Lion-O (alerted by the Sword of Omens) must now brave the Tower's potentially lethal traps in order to rescue Wilykat.

Chilla freezes Snarfer's spaceship as he heads back to the Tower of Omens with Berbil Mexican takeout. When Mandora gets involved, Chilla freezes her and becomes a fugitive from the law as Mandora sets her sights on bringing her in.

Tired of constantly being bested by the Thundercats, the Mutants have Mumm-Ra raise their spaceship and salvage their vehicles.

The Mutants, once again searching for a way to destroy the Thundercats, recover some more items from their spaceship. They take their ship's engine and convert it into a giant fireball-shooting machine. The mutants terrorize Third Earth with their fireballs, until the Thundercats go to castle Plunn-Darr and destroy their dangerous new weapon.

Mumm-Ra creates device that can make Panthro clones. He embeds the spirit of Hammerhand into his clone and sets it free to destroy Third Earth. The Third Earth inhabitants blame the Thundercats for the terror because they think its the real Panthro destroying their villages and burning their crops. The real Panthro finally encounters his clone and, with the help of Lion-O, manages to destroy it. Enraged about his defeat, Hammerhand destroys Mumm-Ra's cloning device.

Mumm-Ra has a new device, the Babylonian Barbarian Boiler, that he plans to use on Cats Lair. Ma-Mutt fools around too much and Mumm-Ra banishes him from the Pyramid. Later, the Thunderkittens, Snarf, and Snarfer bring Ma-Mutt back to Cats Lair and then Mumm-Ra fires the Boiler's beam at the Lair, which causes it to start melting. Snarfer makes a communicating device (because Ma-Mutt was trying to tell them about the beam), so they can understand Ma-Mutt. Ma-Mutt starts telling them of the Boiler, but Mumm-Ra sees this and through the Cauldron's magic brings Ma-Mutt back to the Pyramid. Then the ThunderCats construct a giant Light-Shield and use it to protect the Lair, but Mumm-Ra just boosts the power on the Boiler, destroying the Shield. Lion-O uses the Cat Signal to push back the beam, which causes the Boiler to explode, and the Signal restores Cats Lair. Mumm-Ra, who was lonely without Ma-Mutt, welcomes his dog back to the pyramid

Lion-O and the other Thundercats arrive and save Lynx-o and the Thunderkittens from Red Eye and Tug Mug. The Thundercats learn more about New Thundera, especially the dangerous places affected by the moons of Plundarr where the Lunataks come from. The Thundercats must develop special equipment and clothing to counter those dangers. Also, they learn that all the snarfs have been turned into slaves and Snarf and Snarfer want to rescue them no matter what the cost. Vultureman learns of the Thundercats plan, so he abandons the other Mutants and takes the Rat-Star. But Slithe has snuck aboard. Right after the Feliner takes off, the Rat-star ambushes them and it seems all over for the Thundercats aboard the Feliner. Lion-O uses the sword to free everyone and along with Ben-Gali and Wilykit on the HoverCat, go after the Ratstar and it crashes on Hook Mt. The Thundercats leave Third Earth's atmosphere and Lynx-O tells him that Mumm-Ra has already located the Sword of Plundarr.

Mandora, on space-patrol, encounters an old pirate ship which doesn't meet inter-galactic standards. She boards the ship, arrests the pirate, and sends the ship to a space prison. Little does she know that she has boarded the Jolly Roger, the home of the evil Captain Cracker. Mandora is captured by the creature. Lion-O intercepts one of her distress calls, and must go into space to save her—easier said than done, as Cracker orchestrates a prison break.

The Thundercats think that Snarf is getting too old, and that he is not up to doing as much as he does. To prove them wrong, Snarf saves the Thundercats from another mess. The evil Crystal Queen, Tartara, captures the Arrietta bird, whose songs enchant the Berbils' harvests. Lion-O, upset with her selfishness, goes off to save the bird, but gets caught and put under an evil spell. Snarf, managing to slip through the crystal queens defenses, saves Lion-O and frees the Arrietta bird.

Mumm-Ra summons the great samurai Hachiman to seek and destroy Lion-O. He tells Hachiman that Lion-O and the Thundercats are destroying his planet. Hachiman and Lion-O confront each other, but their swords refuse to draw on each other. They become allies and hope to destroy Mumm-Ra together. Enraged, Mumm-Ra summons another warrior, a ninja, to sabotage the Thundercats, and retrieve Slithe, whom they have captured.

Wily Kit and Kat, exploring New Thundera, stumble upon an Orb of Gold, and pick it up thinking that it is apart of their Heritage. Possessed by the power of the Golden Sphere of City, the ThunderKittens get greedy, and let the power of the orb overcome them. Mumm-Ra manages to get a hold of the orb, and his powers get four times more powerful. He gets so powerful, he destroys the Sword of Omens. Jar, the scrap metal collector reforges it for Lion-O. He gets so powerful that he challenges the power of the ancient spirits of evil. They lock him out of his pyramid until he surrenders the Orb to them. The spirits of evil let him back in, but destroy the Golden Sphere of City.

Lion-O, Panthro, and the Thunderkittens take the Whisker, a new flying vehicle, to the Churning Rocks to rescue the Thunderian refugees who had called from there. The Thundercats get captured by Two-Time, a robot with 2 heads. Two-Time takes them aboard his spaceship, Dome-Down, and the Thundercats see that he has also captured the refugees. Lion-O defeats Two-Time and the refugees are freed. Meanwhile, Lynx-O, Cheetara, and Bengali spot Mumm-Ra near the Churning Rocks, and inform Lion-O of this. Lion-O, Panthro, and the Thundekittens head to the Churning Rocks and confront Mumm-Ra, who is in the form of Jaga again. They break Mumm-Ra's spell (again) over Jaguara, who had temporarily repaired the Gyroscope. Lion-O defeats Mumm-Ra, but Mumm-Ra reminds them all that eventually the planet will be destroyed without the real controls, which he still has...

On the second day of his trials, Lion-O must race Cheetara through the dangerous lands of Third Earth. Although Cheetara is by far the fastest among the Thundercats, her speed can only last for a short time period. Lion-O takes a shorter but more dangerous route to make up for Cheetara's advantage.

Mumm-Ra awakens the Rock Giant, an ancient monster made of stone which looked like a mountain to the TCats. When it wakes up, Mumm-Ra sends it to destroy Cats Lair. The TCats try and defeat it using the Thundertank, and eventually with the help of the Sword, the giant is beaten.

The Thundercats briefly show Lion-O how the time capsule they recovered from the caveman works. Curious to see more, Lion-O activates it during his night-time watch, and gets caught in its projection rays. The rays send him back in time to Thundera, just before the final tremor that destroyed the planet. Meanwhile, the Mutants have built an evil robot monster that is destroying Cats' Lair. Back on Thundera, Lion-O encounters his father, Claudus, and saves him from a Mutant attack. In return, Claudus gives Lion-O the plans to the robot that was attacking the Thundercats in his own time.

An alien spaceship crash lands on third earth, and it can somehow communicate with Cheetara through her sixth sense. She becomes a part of the spacecraft, and begins to feel what it feels. The Mutants recover the space ship, and begin tampering with it. Little do they know that it is not only affecting the spaceship, but it is affecting Cheetara. In order to save Cheetara, the Thundercats set out to recover the space ship from the Mutants and fix it.

Wilykit and Kat find an ancient harp that has a genie in it. His name is Charr-Nin. Mumm-Ra appears to the genie and tells him that if he destroys the Thundercats, he will be freed from his eternal prison in the harp. Foolishly, Char-Nin agrees, and begins trapping the Thundercats in his harp one by one. Mumm-Ra's plan works, and he betrays Charr-Nin. During their arguing, the Thundercats manage to escape, leaving Mumm-Ra and Charr-Nin battling beneath the earth.

Lion-O and Snarf stumble upon a portable jail cell, and mistakenly open it up. Three of the galaxy's worst villains escape. One of them, a slick thief, manages to steal Lion-O's sword. Lion-O helps Mandora, the inter-stellar police officer that imprisoned the villains, recapture them, and return them to their cells.

Mumm-Ra retrieves the Sword of Plundarr and the Thundercats arrive on New Thundera. While the Thundercats want to get the Sword and treasure, Snarf and Snarfer want to free their fellow snarfs. While the Thundercats sleep, Snarf and Snarfer sneak away, but are discovered and captured by Mumm-Ra. He disguises himself as Snarfer and he leads the Thundercats to where the treasure is located, but it can only be freed by the Sword of Omens. Once the treasure is freed, Mumm-Ra reveals himself and defeats the Thundercats with the Sword of Plundarr. Now Mumm-Ra possesses the Sword of Plundarr and the Treasure of Thundera

An evil hunter lands on third earth in search of prey to hunt. His holographic machine can tell him about all of the Thundercats, including their strengths and weaknesses, and he is able to hunt down each one of them and put them in Thundrainium cells. He captures all of them but Lion-O and Snarf. Jaga warns Lion-O about Safari Joe before he sets out to save his comrades, and Lion-O is able to defeat him and release his friends.

The day of the centennial total eclipse has arrived, and any spell Mumm-Ra casts will last until the next eclipse. He declares a "Day of Decay" on the Cats' Lair, causing it to fall apart.

Mumm-Ra hires Captain Shiner to send a fake distress signal to the ThunderCats. With their new vehicle, the Feliner, the ThunderCats set out into deep space to help the "ship in distress". When they arrive, Shiner captures Lion-O and the Sword of Omens. He gives the Sword to Mumm-Ra, and Mumm-Ra throws it into a black hole. Cheetara returns to Third Earth to get Panthro. They rescue Lion-O, and seize Shiner's ship. Panthro sets a course into the black hole to recover the Sword. They enter, and must battle the robotic city of lost space ships to recover the Sword of Omens and escape.

The ThunderCats race to try to salvage the part they need for the Feliner from the wreck of the Rat Star before the Mutants can complete repairs. Snarf contemplates leaving the ThunderCats forever.

Two Bolkins (sheep-like beings from Third Earth) unleash the ghost of Grune the Destroyer, a renegade Thundercat who once terrorised Third Earth and now turns his attention to the Thundercats.