Lab samples leak from a research facility in Maryland, and human-shaped test subjects mutate into monsters when they are overcome by rage. They ride the Imperial, a train headed north to Canada. Rae and her closest friend Thea climb on, and Rae spots one of the creatures. Thea dies before her eyes, and Rae is impaled as the train derails. Then time rewinds, and Rae wakes at the moment she first stepped onto the train.
Each death drops her back at the beginning, but her pain and terror stay with her. She learns the creatures have a fatal weakness, a core at the back of the neck, and she convinces an officer to rally the survivors. A red-thread monster captures her and holds Thea hostage, forcing Rae to spy on the passengers. Pretending to submit, she moves between human and beast, uses her loops to expose hidden test subjects, and moves toward Elliott, the man who leads them all.
