A young couple on their nightly hormonal romp decide to go to the beach, where they first meet a mysterious man who is parked there. After asking for a light, the couple make their way to some nearby ruins, where in the midst of their passion they discover a hanging woman. Upon investigation, it turns out to be a brutalized mannequin. The mysterious man peels off in his car before they can ask him questions.
Enter couple number two, Christian and girlfriend number 1, who also decide the beach is a good place for a little afternoon delight. They too come upon a woman's body, this time face down, just above the surf. However, it turns out she is a real person, who is in fact not dead. Introducing herself as Barbara and not really explaining how she got there, she quickly disappears as Christian is distracted. Despite seemingly being involved with the girl he's there with, Christian can't seem to get Barbara out of his mind. He finds a flask with the word, "Tucania" on it. Christian tracks it to a boat with the same name, harbored in a local marina. He and his girlfriend join a party aboard the boat, where they encounter Barbara and her current lover, Alex.
Christian abandons his girlfriend and leaves the party with Barbara, upsetting Alex. They drive through a wooded area, where they see lingerie-clad mannequins hanging. The couple winds up back at Barbara's motel. Christian tells Barbara about a terrible childhood incident with his father, and Barbara explains that Alex is not her boyfriend but more of a provider. After some flirtation, Barbara makes an odd request: Christian must shave his beard before the two have sex. He complies. As Christian shaves in the bathroom, he hears a noise outside. He pokes his head out the open window and is assaulted by a man with a gun. The man, Tatum, enters the bathroom and threatens Christian. A brief struggle ensues wherein Tatum is shot and killed. Christian leaves the bathroom and tells Barbara everything. Strangely at ease with the concept of a dead man in her room, she convinces Christian that the best thing to do is run. The pair go to a friend's place, a seaside castle and lighthouse.
Christian insists that his brother Fritz can help, but Barbara insists that no one can. The arguing pair are interrupted by squatters Malcolm and Clorinda. Malcolm taunts Christian with a story about a local murder. Believing news of Tatum's death has reached the news, Christian admits that he's the killer, but Malcolm explains that he was just joking. Confused, Christian retires for the night. Clorinda tells Christian that he knows her, and a possibly hallucinatory rape scene ensues. Christian then goes downstairs to see Barbara. The next day, Barbara is gone. Christian searches her motel room, where there is no body nor weapon, to the marina where there is no boat and he's followed by an ominous man and ultimately back to the tower where he sees Malcolm talking to that same man, Luca. Increasingly going out of his mind. He travels to the marina, and finds his boat anchor missing. Christian spends the rest of the day in the tower. That night, he thinks he sees Tatum on the grounds. Making his way down, he discovers Malcolm dead. Christian leaves in a panic, catching a brief glimpse of an arm at the bottom of the well.
Desperate for a safe place, Christian goes to his original girlfriend's apartment. After spending the night there, he's accosted by Tatum, who is very much alive. Tatum forces him to drive to a quarry, revealing along the way that the plan wasn't supposed to involve killing him, just driving him crazy, but since he's being stubborn about it, he has to die. Attempting to make it look like an accident, Tatum has him drive to the edge of the cliff and exits the car. Quickly turning the tables on Tatum, Christian reverses the car, smacking Tatum with the open door and running him over twice. Realizing that there is a conspiracy against him, he switches clothes with Tatum, puts his necklace around Tatum's neck, shoves the dead body in the car and pushes it off the cliff.
Barbara and Luca arrive to make sure that "Christian" is dead. As they leave, Christian follows them back to his family's factory, currently being run by his brother Fritz. It is revealed that Tatum was right — the plan was to drive Christian mad so that he has no claim on the family fortune. Barbara reveals that she has fallen for Christian and is sick of both Luca and Fritz because Christian wasn't supposed to die. The two men are satisfied, though, and tell her to shut up and go along with it.
Christian wanders the roads, disguising himself as a male prostitute and getting picked up by a female client.
After leaving the factory, Luca and Barbara are planning their getaway. Christian interrupts them and reveals that he knows about the plot. Barbara admits to everything. The two reconcile and make plans to escape. As the pair lovingly make up, Christian begins to see Clorinda's face, his ex-girlfriend's face and the random woman who picked him up for prostitution. He chokes Barbara to death and runs away. Fritz meanwhile has investigated the car crash and figures out that it's not Christian's body in the wreckage. Retreating to his mansion, he decides to indulge in some old family movies. As he's watching them, its revealed that Christian had mental problems as a child, and that his father's lunacy is hereditary. As he hears that, he begins to have flashbacks. It is revealed that Clorinda was the nurse at the psychiatric hospital and that Malcolm was the doctor. Additionally, its shown that he has killed Clorinda after the potential rape and dumped her in the well, that he killed the ex-girlfriend, the random woman in the car and Barbara. Alex shoots him, fatally wounding him, but Christian manages to run away. As he bleeds out, Christian makes his way back to the beach where he originally found Barbara and dies.
Fritz wanders home, and makes his way to a closet full of lingerie-clad mannequins, all with puncture marks and other disfigurements. As he slowly makes his way to a particular mannequin, we hear the voiceover again telling us that the insanity is hereditary as he repeatedly stabs the mannequin, releasing his pent up rage and craziness.Robert Hoffmann as ChristianSuzy Kendall as BarbaraIvan Rassimov as FritzAdolfo Lastretti as TorresMonica Monet as ClorindaGuido Alberti as MalcomFranco Silva as Luca
In the original cut, Lenzi opted to not show the murders as to add suspense and mystery about the motives and the identity of the assassin. The American producers felt the audience could be confused, so they added about ten minutes of footage displaying the murders and clarifying some parts of the plot. Reportedly, George A. Romero was hired to shoot the additional footage.