When every single person over the age of 15 mysteriously disappears, the children are left to fend for themselves. While some turn to looting, Sadiq leads a small group to the safety of the City Hotel.
Callum's new gang intend to take over the City Hotel, using Kat to gain access. Meanwhile, Reese visits the library to learn more about the Sparticle Project, and sees 'ghosts' of the adults there.
While visiting the university to find food, Reese discovers a message from Doomsday Dora explaining how the adults disappeared and urging the children to help them return. Meanwhile, Holly is held hostage by the only remaining adult.
To decide who will lead the quest to reunite the two worlds, the tribe compete to find a new power source. It becomes more urgent when they discover the hotel's boiler is about to explode unless the electricity can be restored.
While taking a break at a funfair, Reese has visions of Doomsday Dora who is trying to give them a message. Back at the City Hotel, the national water supply has failed and the team must find a new source of fresh water.
Dora's clue leads the Questers to a science facility where the Sparticle Project was developed. But the simulator leaves Ami, Jordan and Holly trapped in a room with the particle collider's cooling system as the temperature drops.
Ami is taken hostage by a fun-loving group living on a barge when they mistake him for a member of a rival tribe. Jeffrey needs new batteries for his wheelchair so he can help out more at the City Hotel.
While stopping to repair their van, the Questers get caught up in a 'revolution' at a posh boarding school, where rebels are trying to overthrow the Head Boy who will do anything to keep things running as he believes they should be.
The Questers replenish their food supplies at a seemingly abandoned farm, but are caught red-handed by the farmers who accuse them of stealing. Meanwhile, a group of hungry girls follow Liam back to the City Hotel and demand to be fed.
As the tribe get closer to the Sparticle Project, Holly wonders if she really wants the quest to succeed, and leaves her fellow Questers trapped in an underground nuclear bunker.
Without the adults to keep things running smoothly, nuclear power stations start leaking deadly radiation. The Questers must find a way to get a message to the adults.
Ami desperately needs insulin for his diabetes, but the youngsters now running the hospital don't take kindly to the visitors. Meanwhile Tia worries that her approaching 15th birthday could mean she will disappear like the adults.
The two parts of the tribe are reunited at the Black Tor Mine for the final stage in their quest. But another tribe is guarding the Sparticle Project and won't let anyone in.
