In Book 3, Grace and Simon lead The Apex, a rebellious group of kids aboard an endless, mysterious train. During one of their destructive missions, Grace and Simon get separated from the rest of the group and must find their way back. As the duo navigate through the myriad worlds of the train, they meet Hazel, a sweet, optimistic young girl, and Tuba, her imposing but gentle gorilla companion. Will Grace and Simon find new inspiration in Hazel's innocence, or will the insidious ways of The Apex lure yet another wide-eyed recruit?