THE CASE OF THE BLOODY IRIS

A prostitute, Luna, is buzzed into the high-rise apartment building of a regular client. She is stabbed to death in the elevator by a masked killer who vanishes before it reaches the top floor, where the tenants (violinist Professor Issacs, widow Mrs. Moss, and casino stripper Mizar Harrington) discover the body. The police are called as Mizar leaves for her shift at the casino.

Meanwhile, the building's architect Andrea Antinori hires a photographer to find models for advertisements. He becomes enthralled with Jennifer Langsbury, who is posing with her friend Marilyn Ricci. On her way out of the studio, Jennifer is accosted by Adam, the leader of a sex cult named “Iris” she was a part of before escaping into modeling. He sternly warns her if she doesn't come back to him, there will be consequences.

Mizar returns to her apartment, where she is drowned in the bathtub by the killer. In the morning, police commissioners Enci and Renzi investigate the murder. Mrs. Moss is interviewed as a witness. At the crime scene, a love letter is found.

To entice them to star in his advertisements, Andrea sublets Mizar's now-vacant apartment to Jennifer and Marilyn. He joins them for dinner and is disturbed by the sight of Jennifer cutting her finger, revealing his fear of blood. On his way out of the building afterwards, Andrea is confronted by Adam, who tells him Jennifer can't belong to anyone else.

Close to dawn, the killer enters the apartment by the balcony and wraps their hands around Jennifer's throat, but flees when she awakens and screams. Marilyn thinks she dreamed the encounter. The next morning at the drugstore, they meet their neighbor Sheila Issacs, who lives with the Professor, her father.

Andrea is interviewed by the police as a potential suspect. Enci orders Renzi to keep watch on him. Jennifer finds an iris near her door and enters the apartment, where Adam attacks her. She escapes and calls Andrea. The two go out to eat and take a walk in a park, being followed by Renzi.

Jennifer returns to the apartment that night to find the killer waiting. She hits them with a lamp and runs to Sheila's apartment for help. When they go back together, the killer is gone. Marilyn arrives and finds the blood-soaked iris on the floor before opening a closet and discovering Adam, stabbed to death.

Jennifer tells Enci and Renzi about the cult and they assure her she will not be charged in Adam's death. Later, she thinks she hears Mrs. Moss talking to a man in the apartment next door. A handwriting sample test comes back on Mizar's love letter, revealing it to have been sent by Sheila. Enci interviews her, and she comes out as a lesbian (she was also the one who hired Luna at the beginning of the film).

Andrea invites Jennifer to his house, where he proclaims his love for her. The next day, Marilyn is stabbed in a crowd outside the high-rise by the killer. She manages to grab Andrea, who was waiting for a taxi nearby, before collapsing and dying. Frightened by the blood, he flees the scene and is chased by Renzi, but evades capture. Despite Enci's suspicions, Jennifer is sure he is innocent of the murders.

She returns to the building and hears Mrs. Moss talking with the male voice again. She breaks into the neighboring apartment and is attacked by Mrs. Moss' son David, who is deformed and lives in a hidden room. Mrs. Moss arrives and kicks her out. Jennifer calls the police, but Mrs. Moss has hidden David when they arrive and claims she lives alone.

Jennifer receives a call from Andrea, asking to meet him in a junkyard outside of town. She goes there, but Andrea is scared off by Renzi, who followed her. Jennifer goes back to the apartment, runs into Sheila, and the two board the elevator together. However, it goes down to the basement by itself. They exit and explore the room before Sheila is burned to death by a jet of hot steam from a boiler sabotaged by the killer.

Jennifer hides as someone stalks her through the basement. Andrea reveals himself and claims his innocence, but she doesn't believe his words. The police arrive and chase him, but he escapes. Meanwhile, Mrs. Moss finds David missing from his room. Professor Issacs is distraught over his daughter's death and plays the violin in mourning.

Jennifer decides to leave the building once and for all. While packing her bags, she is attacked by the killer again. She goes to Professor Issacs' apartment for help, only to find David's corpse in a chair. The killer unlocks the door and takes off his mask, revealing himself as the Professor. Unable to cope with Sheila's lesbianism, he blamed her sexuality on the other women in the building and murdered them for revenge, killing Adam for interfering with his plan. He modeled his disguise after David's, who wore a similar one while acting as a peeping tom on the other tenants, and plans to frame him for the crimes. He accidentally killed his daughter with the steam, mistaking her in the dark basement for Jennifer.

The Professor chloroforms her and throws David's body down the open stairwell before preparing to do the same to her. Andrea exits the elevator and attacks, managing to overpower the Professor and throw him over the railing to his death. Him and Jennifer share a passionate hug, happy the ordeal is over.