Dr. Alan Feinstone is a Los Angeles dentist who, despite his professional success, quietly suffers from extreme obsessive–compulsive disorder and a persistent fixation on cleanliness and perfection. On his wedding anniversary, he uncovers that his wife Brooke is having an affair with the poolman, Matt. After the moment ends, he arms himself and shadows Matt to Paula Roberts’ house, a friend of Brooke’s. To cover his plans, he fabricates a story of a surprise party and watches Paula invite Matt inside. When Paula’s dog lunges at him, he shoots it in self-defense before returning to his car and heading to the clinic.
Back at the dental practice, the day’s first appointment spirals as he hallucinates a child patient with rotting teeth and accidentally injures him. While Detective Gibbs begins to investigate Paula’s dog’s death, Alan treats April Reign, a beauty queen who becomes his second patient. He hallucinates that she is his wife and, while she’s unconscious, he undresses her and fondles her before attempting to strangle her. When she awakens, he snaps out of the vision and hides her pantyhose. He tells her manager, Steve Landers, that she’s still dizzy from nitrous oxide. Steve quickly realizes what happened, punches Alan, and threatens a lawsuit. The day ends early as Alan sends everyone home, including Sarah, a teenage patient who wants her braces removed.
That night, Brooke arrives at the practice to meet with Alan in a new opera-themed room. Under the pretence of a teeth cleaning, he sedates her and then removes her teeth and cuts off her tongue. The next morning, Detective Gibbs and Detective Sunshine arrive to question him about the disturbing events. After they leave, Matt discovers Brooke—alive but still sedated. Alan responds by stabbing Matt to death.
Sarah and Paula wait for him at the clinic, and Alan’s first glance at Paula leaves Sarah disappointed. Paula talks about Matt’s effectiveness, and Alan aggressively drills into Paula’s tooth, destroying it. His assistant Jessica questions his behavior, and he snaps out of the trance. He asks Jessica to finish, but she reveals she’s sent Paula home and he fires her. When Jessica threatens to expose the crime by pulling out April’s pantyhose, Alan kills her.
At the police station, Detective Sunshine learns that the bullet taken from Paula’s dog matches only one gun in the area: Alan’s. An IRS agent, Marvin Goldblum, tries to leverage Alan’s tax troubles for a free dental exam and payout, but Alan tortures him instead. The detectives rush to the Feinstone house and discover Matt’s body by the pool and Brooke, mutilated but alive, tied to the bed. Alan’s other assistant, Karen, discovers Marvin still in the dental chair; he is then killed by Alan when he attacks her with a needle full of air.
After Alan unwraps Sarah’s braces, he envisions her teeth rotting. She escapes and hides in a dental room where Marvin, bloodied, attacks him. Alan recaptures her, and Sarah pleads with him to brush her teeth three times a day and avoid candy; he relents briefly. The detectives arrive, but they’re too late to arrest him.
They pursue him to a university where he is teaching dentistry. In a frenzy, Alan directs his students to pull their patients’ teeth, and he hallucinates, firing at a dental student he mistakes for Matt. The detectives burst in as he escapes with a hostage, and he wanders into an auditorium where an opera singer practices. The singer morphs into Brooke and taunts him as he reaches out. Defeated, he falls to his knees and is arrested by the detectives.
Now institutionalized in a psychiatric hospital, Alan is wheeled away for a routine dental appointment. The dentist working on him is revealed to be his toothless wife Brooke, who applies brutal, violent pressure to his mouth.
