Dr. William "Bill" Harford and his wife Alice live in New York City with their daughter Helena. At a Christmas party hosted by patient Victor Ziegler, Bill reunites with old medical school classmate Nick Nightingale, who now plays piano professionally. An older Hungarian guest attempts to seduce Alice, while two young models try to seduce Bill. Host Victor interrupts with news of an overdose by Mandy, a young woman Victor was having sex with. Bill aids in Mandy's recovery.
The next night, while smoking marijuana, Alice and Bill discuss their unfulfilled temptations. Bill is not jealous of other men's attraction to Alice, believing women to be naturally faithful. Alice admits to fantasizing about a naval officer she met on vacation and considered leaving Bill and Helena. Bill is disturbed before being called to a patient's house. The patient's daughter, Marion, tries to seduce Bill, but he resists.
After leaving Marion's, Bill meets a prostitute named Domino. When Alice calls, he pays Domino for a non-sexual encounter and meets Nick at a jazz club. Nick describes a masked orgy in a mansion outside New York City at which he will play piano blindfolded, and gives Bill the password to enter the party. Bill goes to a costume store owned by a patient of his in order to buy an outfit to fit in at the masked orgy. Finding the costume store now owned by a man named Milich, he offers money to rent a costume from the shop, where he and Milich find Milich's young daughter with two men.
Bill goes to the mansion and gives the password, discovering a sexual ritual in progress. A masked woman warns him he is in danger. He is brought before the master of ceremonies who demands to know a second password for the house, revealing that the password Bill has is only to enter the grounds. Bill removes his mask at the demand of the master of ceremonies, but the woman who warned him intervenes. She insists on redeeming him, at a personal cost. Bill is let off with a warning to keep quiet.
Bill comes home feeling guilty and confused, only to find Alice laughing in her sleep. He wakes her up and she tearfully tells him about a dream where she was having sex with the naval officer and many other men, and laughing at the idea of Bill watching. The next day, Bill goes to Nick's hotel, but the desk clerk tells him that Nick left with two dangerous-looking men. Bill returns the costume, but realizes he has misplaced the mask, and learns that Milich has sold his teenage daughter into sex slavery. Milich implies that Bill can pay to have sex with his daughter if he likes.
In the afternoon, consumed by thoughts of his wife's infidelity, Bill leaves work early and returns to the site of the orgy. At the front gate, he is handed an envelope with a warning to stay away. That evening, Bill tries to call Marion, but hangs up when her fiancé answers. He decides to go to Domino's apartment to consummate their affair, but is met by her roommate, Sally. Although there is sexual tension between them, Sally informs Bill that Domino has just received news that she is HIV-positive. Bill leaves.
After leaving the apartment, Bill is followed by a mysterious figure. He discovers from a newspaper article that an ex-beauty queen has died from an overdose and identifies her as Mandy at the morgue. Later, Ziegler summons him and admits to being a guest at the orgy. Ziegler reveals that there was no second password at all, and failing to know this is what outed Bill as an outsider. Ziegler assures Bill that the secret society only aims to intimidate him into silence but implies that they are capable of taking action if necessary. Bill is concerned about Nick's disappearance and the death of Mandy, whom he correctly identifies as the masked orgy participant who sacrificed herself for him. Ziegler claims Nick is safely back with his family in Seattle and that Mandy died from an accidental overdose due to drug addiction and not through any foul play.
Bill returns home to find the rented mask on his pillow and confesses to Alice about the past two days. The next day they go Christmas shopping with their daughter and Bill apologizes to Alice. She suggests they do something "as soon as possible," to which Bill asks what she means and Alice simply responds with one word: "Fuck."
