The Sparticle Mystery

When a group of children on a school visit to some caves return to the surface, every single person over 15 has disappeared. Opportunistic gangs roam the streets as Sadiq leads his newly-formed tribe to the safety of the City Hotel. After discovering her mum and sister missing, young scientist Kat takes charge of Liam, the boy next door. They too try to gain entry to the City Hotel, but only Liam gets in, leaving Kat to fend for herself.

Callum has formed a biker gang and has designs on the City Hotel as the gang's new hang out. He befriends Kat as she might be useful to his success. However, Kat is on her own mission to find out what has caused the disappearance of all the adults and encounters Reese at the City Library. Reese insists she can see the adults and feels she has found an ally in Kat. Meanwhile, inside the hotel, Holly is left wondering where her parents are.

Having won her place in Sadiq's gang, Kat leads a team to the University to find food. Holly is taken hostage by what appears to be the only surviving adult. Reese discovers a message from Doomsday Dora, explaining her heart theory - which also explains the curious survival of the adult who is actually - at heart - a child. The need to generate new sources of fresh food leads Sadiq to free animals from a city farm. This goes very badly until Tia discovers that the cheap perfume she wears is a magnet for the nanny goat.

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.