Traveling through a barren, otherworldly wasteland, the Infinity Train is full of puzzles and perils. For those in mental turmoil, they can climb aboard and suddenly find themselves wandering its length where each car is a whole other world. Each passenger has his or her own story that led them onto the train, but once aboard, the goal is both simple and difficult: by solving the mysteries of the train and interacting with its strange denizens, solve one's own mental troubles and through it hopefully find their way off the train.
13-year-old Tulip finds herself on a mysterious train with an endless number of cars, each one its own universe, and must find a way to get home.
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?