Samples leak from a Maryland research lab, unleashing hordes of human-shaped test subjects that mutate when overcome by rage. The creatures board the Imperial, a train bound for Canada.
Teen Rae and her best friend Thea get on the train, where Rae spots Dane, one of the monsters. Before her eyes, Thea is killed, and Rae is impaled by tentacles. Just as the train derails and crashes, time rewinds—Rae returns to the moment she first boarded.
She soon learns every death sends her back to the start, yet all agony and terror stay etched in her memory. Determined to save Thea and every passenger, Rae sacrifices herself repeatedly. She discovers the creatures’ fatal weak spot: a core at the nape of their necks, and convinces Officer Carter to rally survivors to fight back.
A red-thread monster named Mrs. Miller captures them, taking Thea hostage to force Rae to spy on passengers. She implants Crimson Thread inside Rae’s body. Rae pretends to submit, navigating the line between humans and beasts. Using her time loops, she uncovers hidden test subjects and closes in on their leader, Elliott.
Larry, a suspected ally, turns out to be a traitor who uses human survivors to eliminate rival monsters. Rae manipulates Larry to slay Mrs. Miller and key creatures, then kills Larry with foresight honed from hundreds of deaths.
Elliott wields combined mutant powers. After 777 fatal loops, Rae fails to defeat him—until a forced injection merges her fully with the Crimson Thread within her.
In the blizzard finale, Rae slays twin guards and confronts Elliott atop the speeding train. Realizing killing him will send all carriages plummeting off a cliff, she severs the engine from passenger cars to let the rest brake safely. Trapping Elliott in the locomotive, she rides it down to a fatal explosion, saving everyone on board.
Later, Blake retrieves a writhing piece of the red thread from the wreckage, leaving Rae’s fate unknown.
