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.