Doomsday Dora's message has warned the children they must reunite the two parallel worlds quickly or adults and children will remain separated permanently. The ticking clock has started. The children decide to choose who goes on the road to the Sparticle Project and who stays behind to look after Liam and Frankie by an 'Apprentice'-style challenge to source power for the City Hotel.

Jordan, Holly and Ami take being kids way too far, and nearly kill themselves on a runaway waltzer, until Muna, a child caught between the parallel worlds, comes to the rescue. The lack of grid power has led to the failure of the national water supply. Fresh water becomes the latest challenge for the Ranchers, and Frankie goes native in her attempts to find and save the precious resource.

A postcode given to Reese via Psychic Sarah at the fairground leads the Questers to a simulator where scientists were trained how to run the Sparticle Project. Ami, Jordan and Holly get trapped in the room with the prototype collider's super-cooling systems running, and the temperature is plummeting towards absolute zero. At the City Hotel, a fox has been at the chickens and the children decide to trap it. This is very tricky, but when they finally track down the animal and are set to destroy it, they decide on a better course of action.

The Questers are intrigued by Head Boy Jeremy and his posh boarding school. However, they are soon locked out and Jordan has left the tribe to become a true public school believer. Camped outside the school, ex-pupils Ernesto and his rebel gang recruit the Sparticles to overthrow Jeremy's regime and rescue the Unsuitables - the 'less-than-perfects' who do the dirty work for Jeremy and his cadets. The Ranchers at the City Hotel work out how to bypass the first door at the Sparticle Project. Working with Anita at the university, they come up with an ingenious solution, but now they need to get the key to the Questers out on the road.

Jeffrey is proud of his abundant garden, but when hungry waifs turn up at the hotel, he finds himself at war. The Questers on their way to the Sparticle Project discover a bucolic village run on communal principals. Reese 'sees' Muna again and with the help of Tamsin, one of the village kids, Kat figures out that the wormhole events are occurring at points where leylines intersect. Some of the Questers, however, have been caught red-handed stealing food and are made to stand trial in the children's court where the issue is whether stealing to survive is justified.

The Ranchers have invited a tribe of seriously idle girls into their hotel. As they take advantage, the Ranchers are forced to fight back - but who will be turfed out of their home? Holly is anxious when she realises the journey to the Sparticle Project is coming to its conclusion. She leaves her fellow Questers trapped in an underground nuclear bunker, but there is another force at work. A computer who thinks there has been a nuclear war competes against the ingenuity of the children in a battle of wits.

The Questers discover an alarming new threat. Without the adults, and the automatic cooling systems, the world's nuclear power plants are melting down and threaten to destroy the earth. The Questers have to get a message to the adults - but how? Callum arrives to collect the key to the Sparticle Project from the Ranchers, only to discover them homeless. Tia and Jeffrey don't trust Callum after past encounters. But the key has been lost and sold and Callum is the only hope to getting it back and to reuniting them with their friends.

Ami lapses into a diabetic coma and the Questers rush him to hospital to find some insulin. Switching a generator on in the hospital may just have cost one baby his life. The kids running the hospital need fuel to run the generators to keep the babies alive. Aware of the ticking clock to get to the Project but desperate to put things right, the Questers undertake a dangerous mission as a solution. They have to distract a German shepherd dog who is guarding a yard full of petrol lorries. Meanwhile, it is Tia's fifteenth birthday. Will she be transported to join the adults in a parallel dimension?