In the post-apocalypse, we meet our heroine, a psychically powerful girl named Ellie, 12 years old, with her pet, a monkey named Puck. She is traveling in space back to earth, to London, in a space pod. We learn she was abducted by space pirates, and back home, her parents were told she was killed. Her kidnapper is Mal Gorman. We learn that he did not intend to free Ellie, and now that she has escaped, he plans to recapture her.
Ellie is smart though, and she evades him and makes her way back home. They are not excited to see her, partially because life in London is also incredibly dangerous, still. Her brother missed her greatly, she learns, and he is mad that she was gone for so long. We are told about a giant concrete wall, 50 feet tall, that cut off Ellie's community from resources. Ellie avoids the yellow gas that is in the air—it is a poison that humans used to eliminate all other animals from the planet. The brother, Mika, has psychic powers.
Gorman re-abducts Ellie along the River Thames. He intends to use her psychic abilities for his own interests. Separately, Mika contacts Gorman himself, to ask about his sister, but Gorman tells him what he told her parents previously, that she is dead. But, during an uprising, the siblings are reunited: They escape in a police air craft, over the wall. They learn that there is no yellow gas past the wall.
