At a chic 'haute couture' fashion house in Rome, model Isabella (Francesca Ungaro) is brutally murdered one night during a raging thunderstorm by a mysterious figure dressed in black, wearing a mask. The body is discovered locked in a storage locker by the owner: the recently widowed Countess Christina Como (Eva Bartok), which is managed by Massimo Morlacchi (Cameron Mitchell). Inspector Silvestri (Thomas Reiner), an arrogant police detective, arrives to take charge of the murder investigation. Silvestri soon discovers that the fashion house is a hotbed to drugs, corruption, and blackmail. The suspects include Massimo, the wealthy Ricardo Morellin (Frnaco Ressel), who was having an affair with Isabella, the dress designer Cesar Lazzarini (Louis Pigot), and Isabella's boyfriend Frank (Dante Di Paolo) an antique shop owner who sold cocaine to some of the models on a daily basis, as well as the handyman Marco (Massimo Righi).
The following evening during a fashion show, when it is discovered that Isabella kept a diary which detailed most of the indiscretions, everyone is thrown into a quandary. Initially, the diary falls into the hands of Nicole (Ariana Gorini), one of the models, who promises to take it to the police. But another model, named Peggy (Mary Arden), manages to obtain it unnoticed. Later that night, Nicole goes to visit Frank at his antique shop. While there, she is terrorized by strange sounds and fleeting shadows. Before she can escape, she is attacked and killed by the same mysterious figure dressed in black who whacks her in the face with a clawed antique mallet. Finding that the dead girl doesn't have the diary on her, the killer steals Nicole's car, setting off the burglar alarm, and drives away. The killer is witnessed by a gas station attendant nearby.
Across town, Peggy arrives at her apartment from the fashion show where she lives with Nicole and Tao-Li (Claude Dantes). While Tao-Li goes off to a party, Peggy is met by boyfriend Marco. Peggy tells Marco that she doesn't want to go out with him tonight, and he leaves. Alone, Peggy reads some of Isabelle's diary which a page details her blackmail dealings with a mysterious boyfriend. Peggy then burns the diary by throwing into her fireplace. The killer arrives, and kidnaps Peggy in her apartment. Refuses to believe that she burned the diary, the killer takes Peggy to the apartment building's basement and proceeds to torture her. But when Peggy grabs the killers mask (we don't see his face) he puts his mask back on and kills her by burning her face on a heated red-hot furnace.
Meanwhile, Frank arrives at his antique shop and discovers Nicole's dead body. He flees without notifying the police to Ricardo Morellins' house to ask him to supply an alibi for him. Morellin refuses, but Frank threatens to expose him for stealing money from Isabella. Frank calls Inspector Silvestri, who arrests him on suspicion of killing Nicole, as well as dealing drugs. Frustrated, Silvestri has all the five suspects, Massimo, Marco, Morellin, Frank, and Caesar arrested.
But that evening, Greta (Lea Kruger), the fiancée of Morellin, drives home where in the truck of her car, she discovers the burned body of Peggy. Fearing the police will think she did it, Greta carries the body inside her house, where she hides it. Greta soon gets murdered by the same black-clad figure who breaks into the villa who smothers her with a pillow.
The next morning, Silvestri arrives at the villa where he concludes that since the murder was committed with all the suspects in jail, none of them can be guilty. After Massimo returns home, he is greeted by Christina where its revealed that Massimo really is the killer after all. Massimo had previously assisted Christina to murder her husband so she could inherit the fashion house. But apparently Isabella discovered this and began blackmailing Massimo, among other people with scandalous secrets. When her blackmail demands became too much, he murdered her, not realizing that she had written everything down in her diary. Although Massimo murdered Nicole and Peggy too, it was Christina who dons the killer's black coat and mask to murder Greta to throw off suspicion from her lover. Yet, Massimo knows hes not out of danger yet. So, he asks Christina to don the mask and coat one last time to murder another model to further frustrate the police.
That evening, Christina, wearing the mask and black coat, surprises Tao-Li in her apartment and forcibly drowns her in the bathtub, and subsequently slits her wrists to make it look like a suicide. As Christina prepares to leave, she is startled by a knock on the front door. She attempts to bypass the visitor by escaping by the roof. But Christina loses her footing on the ledge, and falls to the ground. The visitor is Massimo, who has instigated a cunning plan to lure Christina into a trap where she will be killed while killing another model, and make it look that shes is the real killer, while he takes over the fashion house for himself.
Confident that all his problems have been solved, Massimo sets about looting Christinas valuables. While in the process of doing so, he is startled by a noise. Looking up, he sees Christina standing before him. Massimo is dumb-struck and Christina explains, the show awning broke my fall. Bloodied from a head wound and trembling in pain, she confront her worthless lover with his treachery, and he naturally tries to smooth-talk his way out of the situation. As they embrace, she realizes his true nature when she sees him pulling a knife out of his coat pocket to stab her in a mirror reflection, and she shoots him to death. Subsequently, as she tries to phone Inspector Silvestri, Christina succumbs to the fatality of her wounds.