In Book 3, Grace and Simon are the leaders of the Apex, an anarchic group of kids on a mysterious train. During one of their destructive missions, Grace and Simon get separated from the rest of the Apex and must find their way back. As the duo navigate through the myriad worlds of the train, they meet Hazel, an optimistic young girl, and Tuba, her gentle gorilla companion. Will Grace and Simon find new inspiration in Hazel's innocence, or will the ways of the Apex recruit yet another aboard the train?