In the quiet town of Peacock, John Skillpa (Cillian Murphy) with a dark secret. Most of the town's residents see him just as a quiet, ordinary bank clerk living alone in a house at the edge of town. Nobody in town knows that John suffers from multiple personality disorder; disguising one half of his personality under the character of "Emma". One day, a freight train derails and crashes into his back garden, unleashing a whirlwind of events. As John realises that his alter ego is under threat of discovery he invents a story involving a secret marriage in order to maintain his dual life. However, "Emma" doesn't want to remain a secret anymore, and she begins to take over his life completely; ignoring his wishes and seeking out desires of her own.