Dianne Bates and her sons Jason and Bret come to the underwater base ORCA for six months. They meet Winston Seth which is Dianne's assistant. Sher is going to study whales. Brett finds the place cool, but Jason donesn't like it. They also meet commander Lucas. At dinner Brett meets his new friends Zoe and Froggy whos real name is Jake Reilly{}. Dianne, Winstorn, Jason and Brett go out with the boat to put a transmitter on the whale. A girl comes up out of the ocean and stops Jason when he is about to shoot the transmitter on the whale, so Dianne has to do it instead. Nobody believes Jason when he tells them about the mermaid.

Vanessa Lane comes to ORCA and ends up in the same class as Jason. Brett, Zoe and Froggy play a trick on Vanessa and she gets back at Brett and leaves him in a broken boat out at sea. A storm comes up, so they can't search for him. He strands on Neri's island and meets Neri, and tells Jason about her when he comes back.

Jason gets his boat license and goes out to sea to sink a drift net. Charlie gets stuck in the net and Neri tries to save him, but gets stuck also. Brett chases a tree frog in the forest and falls into quicksand. He starts screaming for Jason and the sand sucks him deeper but he is rescued. When Jason realises what has happened, he works against time to free them both and Neri reveals a secret place to Jason.

Vanessa and Jason go out on a mission with the boat but Neri warns Jason that an earthquake is coming, so they return. Commander Lucas gets angry but the earthquake is for real and Damien and Lee get stuck in a room leaking water. Froggy pulls a trick with the computer called HELEN which stands for Hydro Electronic Liaison Entity 3000{} to try to get them out.

Brett's birthday party proves to be a big hit with his friends but Brett is disappointed when he doesn't receive a gift from his father (his parents are divorced). Following a heated argument between Dianne and Commander Lucas over the acquisition of new equipment for Dianne's research, Winston suggests that they should make an effort to be nicer to one another. Talking with Froggy and Zoe about divorce and stepparents, Brett gets the idea that there may be a relationship at work between his mother and Commander Lucas, especially when he sees how nice they are being to each other of late. Brett's belief seems to be validated when he finds himself being given a private tour of the bridge by Commander Lucas, during which he decides to casually mentions some unsettling details about his mother hoping to disinterest the Commander. After saving Jason from a shark while swimming near her island, Neri tells Jason her life story—that which she remembers.

Jason's frequent use of the boats results in Commander Lucas banning him from using them for a period of one week, effectively cutting him off from Neri. A pair of eager developers arrive on ORCA expressing an interest in a nearby island—Neri's. Jason volunteers to be their guide on their visit to the island and once on the island he and Neri are able to scare off the developers keeping the island safe. When Jodie takes forty dollars from a lost wallet, Vanessa forces her to spy on Jason and Brett in order to keep the theft a secret.

Neri brings Jason's attention to a toxic dump in the ocean that is making Charlie sick. When Jason is reprimanded for leaving an important project, it is up to Lee, Brett, Froggy and Zoe to prove to Commander Lucas that the dumping of waste in the ocean is occurring. A barrel is recovered and dramatically revealed during a presentation on ORCA by Zenco, a major sponsor of ORCA research and the company responsible for dumping the waste. Neri once again expresses her desire to see the inside of ORCA.

Jason agrees to show Neri his and Jason's home on ORCA. After steeling one of Vanessa's uniforms and obtaining a fake ID from Froggy, Brett begins a long-winded tour of ORCA that he hopes will bore Neri into making her visit a short one. Neri however is quickly captivated by ORCA—just as Damien seems to be with her. Getting separated from Jason and Brett, Neri wanders into Dianne's lab where she encounters both Dianne and Commander Lucas. Meanwhile Vanessa discovers that one of her uniforms is missing and has an idea of just who is wearing it. Fortunately Jason and Brett are able to get Neri away from an insistent Vanessa and off of ORCA before any more trouble arises.

As the kids are preparing for their shore leave to Port Jackson, Neri makes a surprise visit that forces Jason and Brett to take her with them. Damien invites Neri to see a movie but the experience frightens her and she runs off. On her own, Neri meets a young boy and helps him retrieve a stolen bracelet. Returning to ORCA, Jason and Lee find themselves alone but Commander Lucas' arrival makes for an awkward moment. Neri asks Brett what kissing is like and when he suggests she try it, she does—with Damien. Dianne and Winston come to the conclusion that Charlie is actually communicating with the small animal that accompanies him (Neri) but Dianne is not happy by Jason's lack of interest in the success of her research.

New monitoring equipment allows Dianne and Winston to hear Neri's communications with Charlie but when the tag on Charlie stops working Dianne must go out and replace it. Using the songs of Charlie and Neri, Dianne and Winston are able to attract Charlie to their location. Dianne runs out of oxygen while diving but is rescued by Neri who is forced to reveal herself to Dianne and Winston in the process. When Dianne's research is faced with being shut down, Neri replaces the lost transmitter on Charlie enabling her research to continue.

Dianne asks Neri to help with her research on whales, Dianne is especially curious as to how Neri is able to communicate with Charlie. Arrangements are made for Neri to stay on ORCA but as Dianne's work with Neri continues, Jason accuses his mother of ""monstering"" Neri when all Neri wants is to be a part of their family. Brett, Froggy, and Zoe chase after the spy who has been steeling Dianne's whale recordings.

The spy takes the stolen whale recordings back to Dr. Hellegren at UBRI—Hellegren wants to be the first person to communicate with whales. Dianne's tests on Neri reveal a remarkable telepathic ability. Dr. Hellegren uses the stolen recordings to lure Charlie into an isolated bay bounded by an electrified net. Dianne readies Neri for her first dance but upon hearing Charlie's calls for help, Neri rushes out midway through. Curious as to why someone would steal recordings of whales, Commander Lucas decides to look into Dianne's research and discovers all about Neri.

Jason and Brett round up their friends to help rescue Charlie, but Jason must also tell them the truth about Neri. Dianne destroys all of her research on Neri after Commander Lucas tells her that he will have to file a report on Neri. The kids infiltrate the UBRI command post and are able to release the net and free Charlie. A blood test done on Neri indicates that she is not human. Dianne convinces Commander Lucas not to file his report. Leaving with Charlie on his annual migration, Neri tells her friends to watch for her return.