Peter Neal (Anthony Franciosa) is an American writer of violent horror novels whose books are tremendously popular in Europe. In Italy to promote his latest work, entitled Tenebrae, he is accompanied by his literary agent Bullmer (John Saxon) and his adoring assistant Anne (Daria Nicolodi). He is unaware that he is also being followed by his embittered ex-wife Jane (Veronica Lario). Immediately prior to Neals arrival in Rome, a beautiful young shoplifter (Ania Pieroni) is brutally razor-slashed to death by an unseen killer. The murderer sends Neal a letter informing him that his books have inspired him to go on a killing spree. Neal immediately contacts the police, who put Detective Giermani (Giuliano Gemma) in charge of the investigation, along with the detectives female partner Inspector Altieri (Carola Stagnaro).
More killings occur. Tilde (Mirella D'Angelo), a beautiful lesbian journalist, is murdered at her home along with her lover. Later, Maria (Lara Wendel), the young daughter of Neal's landlord, is bloodily hacked to death with an axe after stumbling into the killer's lair.
Neal notices that TV interviewer Christiano Berti (John Steiner) appears to have an unusually intense interest in the novelist's work. At night, Neal and his second assistant Gianni (Christiano Borromeo) watch Bertis house for suspicious activity. Neal decides to separate from Gianni in order to get a better view. Alone, Gianni watches in horror as an axe-carrying assailant brutally hacks Berti to death. But he is unable to see the murderer's face. Gianni finds Neal unconscious on the lawn, having been knocked out from behind.
Giermani's investigation reveals that Berti was unhealthily obsessed with Neal's novels, and now that he is dead it is believed that the killings will cease. However, Bullmer, who is having an affair with Jane, is stabbed to death while waiting for his lover in a public square. Gianni is haunted by the thought that he had seen, but did not recognize, something important at Bertis house during the night of the interviewer's murder. He returns to the house and suddenly remembers what was so important he had heard Berti confessing to his attacker, "I killed them all, I killed them all!" Before Gianni can share this important detail with anyone, he is attacked from the back seat of his car and strangled to death by an unseen assailant.
Jane sits at her kitchen table when a figure with an axe leaps through her window, hacking off one of her arms. She spews gallons of blood over the kitchen walls before falling to the floor, the killer continuing to hack at her until she is dead. Neal is her murderer. Upon learning the details of Berti's sadistic murder spree, Neal had suddenly been overwhelmed by a forgotten memory involving Neal's murder of a girl who had sexually humiliated him when he was a youth in Rhode Island. The memory now constantly torments him and has inflamed his previously repressed lust for blood. Neal has become completely insane. Berti was the killer of all the women by razor, but it was Neal who also killed Berti, Bullmer and Gianni.
When Inspector Altieri arrives at the house a few minutes after Jane's death, Neal kills her too. Later, Giermani and Anne arrive at the house in the pouring rain, and when Neal sees that he cannot escape, he commits bloody suicide in front of them by slashing his neck with a straight razor. Anne runs outside to her car for a moment. Giermani relaxes and is suddenly murdered by Neal, who had faked his own death. Neal waits inside for Anne to return, but when she opens the door, she accidentally knocks over a metal sculpture that impales and kills the demented writer. The horror-stricken Anne stands in the rain and screams over and over again.