After a murder is committed, Will finds himself in possession of a key that has the potential to unlock the train's hidden treasures.

Together with Maren, a gifted escape artist, and Mr Dorian, a circus ringmaster with amazing abilities, Will must save the Boundless before someone else winds up dead.

With villains fast on his heels and strange creatures lurking outside the windows, the train hurtles across the country as Will flees for his life. His adventure may have begun without his knowing . . . but how it ends is now entirely up to Will.