Late one night at the Russell Square station in the London Underground, university students Patricia Wilson (Sharon Gurney) and her American exchange student boyfriend Alex Campbell (David Ladd) find an unconscious man (James Cossins) collapsed on the stairwell. Fearing the man may be diabetic, Patricia checks his wallet and finds a card that reads James Manfred, OBE. Alex and Patricia inform a police officer about Manfred, Alex and the officer return to the stairwell to Alex's surprise find that Manfred has vanished. Inspector Calhoun (Donald Pleasence) is assigned to look into the disappearance. Calhoun antagonistically questions Alex, who asserts the man was a drunk, and suggests he and Patricia robbed the man.While discussing the case of the missing Manfred, Calhoun's colleague Detective Sgt. Rogers (Norman Rossington) tells him about the history of the London Underground, particularly the Victorian railway workers who constructed the tunnels under dire conditions, and an urban legend that a group of descendants who survived an 1892 cave-in still live below ground in an abandoned section of the tunnels.Meanwhile, one of the last surviving members of a family of these railway workers (Hugh Armstrong) watches his sick, pregnant, female companion (June Turner) die; they have survived since birth in the underground by resorting to cannibalism of the railway patrons. In an empty chamber, Manfred's body lies, mutilated beside other decomposing bodies of various victims. The man laments the woman's death, as he is now left in complete solitude. The man goes into a rage and brutally murders three Underground maintenance workers in, taking one to his lair to feed on.Calhoun remains suspicious of Alex and Patricia, and calls Alex in for repeated interrogations. After seeing a film one night, Alex and Patricia take a train home and get off at Holborn station. While de-boarding, Patricia realizes she forgot her textbooks on the train. Alex attempts to retrieve them, but the doors close before he can exit; just as the train leaves, Patricia yells through the window that she will meet him at home. Once the train exits the platform, Patricia is attacked by the cannibal man and incapacitated.When Patricia fails to meet him at their flat, Alex seeks help from Calhoun, who is dismissive of him. Alex returns to Holborn station to search for Patricia, and enters the tunnel against the orders of a station attendant. He manages to breach an abandoned area of the Underground that had caved in, and finds the skeletal remnants of the miners who worked there over a century ago.Meanwhile, Patricia awakens in the cannibal's lair. She finds him to be aphasic and unable to communicate with her. When he begins grabbing her aggressively, she hits him over the head and manages to flee, escaping into a tunnel. He finally corners her and attempts to communicate with her, speaking in the only line that he knows: "mind the doors"- having heard the phrase repeatedly from train conductors at the tube stations. But the man becomes frustrated when Patricia does not reply and he violently attacks her.Alex stumbles upon the scene and begins fighting with the cannibal, stomping him on the head. Patricia, however, begs Alex not to hurt him, and they watch as the cannibal stumbles into a passageway. Calhoun, Rogers and several other police detectives who have entered the Underground discover Alex and Patricia. As they search through the abandoned section of the Underground tunnel, the detectives uncover a room full of corpses laid in bunk beds - the generations of survivors from the cave-in that occurred a century before. There they find the cannibal, bleeding profusely, and he collapses in front of them, apparently dead. The detectives return meet Alex and Patricia, who have been waiting outside the cannibal's lair, and they all head to the station platform to call in backup and a forensics crew. After they leave, the cannibal screams "Mind the doors!" revealing that he is still alive.End credits