Two London sewage workers, Arthur Ken Campbell and George Vas Blackwood, discover a hidden tunnel behind a wall. Arthur, the more curious one, slips into the darkness to explore, while George hesitates at the threshold. After a while, George follows and finds Arthur badly injured, immobilized and in shock. A second figure, an injured woman, bursts out of the tunnel crying for help, only to be dragged back into the darkness. The scene hints at something terrible lurking beneath the city.
A young German woman, Kate Franka Potente, makes her way toward a party and heads to Charing Cross Underground station. She dozes off on the platform while waiting for a train, and when she wakes, the station is locked up for the night. An empty train arrives, the car lights flicker, and she boards it. The train stops abruptly and the lights go out. There, she meets Guy Jeremy Sheffield, a coworker who follows her and whom she repeatedly rejects as he makes crude advances. Guy attempts to force himself on her, but an unseen aggressor drags him out of the train. He staggers back briefly, bloody, and warns Kate to run.
Kate flees and encounters Jimmy Paul Rattray and Mandy Kelly Scott, a homeless couple living in a storeroom with their dog Ray Strapper. Jimmy reluctantly agrees to help her by taking her to the night guard after she pays him. They locate Guy, horribly maimed but still alive. Mandy, left alone, is attacked and kidnapped, sending Jimmy into a sorrow-fueled haze of heroin. Kate manages to speak with the security guard via intercom, but the guard is killed before he can call for help, while Guy dies from his injuries. Kate and Jimmy decide to walk through the tunnel to the next station. A train stops near them; Jimmy chooses to confront the killer to avenge Mandy, but he is slaughtered.
Fleeing, Kate falls into the sewer system below, where she finds Arthur’s body. She also discovers a storage facility filled with hundreds of boxes, and she is captured by the killer, Craig Sean Harris, the titular “creep”—a hideously deformed, mentally ill hermit who keeps his victims in semi-submerged, rat-infested cages until they die, after which he eats them. Kate is placed in one of the cages, alongside George, the sewage worker. They manage to escape and end up in an abandoned medical facility, where they find an unconscious Mandy strapped to a surgical chair. Thinking she is dead, they move on. Craig reappears, dons a surgical gown, and mimics the gestures of a surgeon in front of the terrorized Mandy before disemboweling her with a bone saw.
Kate and George locate the disused railcar where Craig lives. The dog Ray is there, along with old photographs of a medical doctor with a deformed child. Craig ambushes them and kills George. In a last desperate move, Kate sinks a hook and chain into Craig’s throat, and a running train tears it free, leaving him to bleed to death. Disheveled, she returns to the initial station as morning light begins to creep in. Ray curls onto her lap. A man waiting for the train, mistaking her for a beggar, leaves her a coin, and Kate breaks into a fit of hysterical giggles and tears.
