The music aspirations of Ryan and Min-Gi are put on hold as they board the train at the same time and receive the exact same number.... Can this friendship withstand the train?

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?

The second installment follows Mirror Tulip, an escaped reflection from the mirror world, and Jesse Cosay, a 14-year-old jock from Arizona. As the pair finds themselves on the run from the mirror police, themes of identity, friendship and peer pressure arise.