Heading home late one night after a party, Kate falls asleep while waiting for her train. She awakens to find herself trapped in the London underground, with all the doors locked for the evening. While being attacked by a co-worker who has followed her, a mysterious unseen creature drags him away and kills him. This begins a terrifying ordeal, as Kate and a young homeless couple are stalked through the dark tunnels by something dangerous with payback on its mind.