Dr. Corbin Bernsen stars as Dr. Alan Feinstone, a highly skilled dentist living in Los Angeles who secretly struggles with severe obsessive–compulsive disorder. Despite his professional achievements, Alan is plagued by intense fears related to cleanliness, order, and perfectionism, which deeply affect his daily life and relationships. His obsession with maintaining absolute control over his environment becomes increasingly apparent as the story unfolds.
On what should be a joyful occasion—his wedding anniversary—Alan is shocked to discover that his wife, Brooke (played by Linda Hoffman), has been unfaithful to him with their poolman, Matt. The revelation sparks a series of disturbing events, starting with his decision to follow Matt in his car. This leads him to Paula Roberts's house, a close friend of Brooke. Under the false pretense of a surprise celebration for his wife, Alan observes Paula inviting Matt inside. The situation quickly turns chaotic when Paula's dog attacks Alan; in a defensive reflex, he shoots the animal, further adding to the mounting distress.
Later that night, Alan returns to his dental practice, where his mental state begins to deteriorate. His first patient of the day is a child, whom he hallucinated having rotten teeth, leading him to inadvertently stab the young patient—a tragic and alarming mistake. As police Detective Ken Foree investigates the death of Paula’s dog, Alan’s second patient, April Reign (a beauty queen), arrives. Alan hallucinates that April is his wife, and in a disturbing episode, he undresses her while she is unconscious, fondles her, and later attempts to strangle her before she wakes up. His behavior shocks her manager, Steve Landers (played by Mark Ruffalo), who witnesses the incident and recognizes the unacceptable violence.
Throughout the day, Alan’s grip on reality continues to slip. He leaves work early, sending his staff and patients home—including Sarah (a teenager wanting her braces removed), which shows his increasing detachment and instability. Later that night, Alan surprises Brooke at his practice in a new, opera-themed room. Under the guise of teeth cleaning, he sedates her and carries out a gruesome act—removing her teeth and cutting off her tongue, a shocking culmination of his obsessive-compulsive rage and desire for control.
The following morning, detectives Sal Viscuso and Tony Noakes arrive at Alan’s house to question him. They find Brooke, still alive but heavily sedated, and at the same time, Matt discovers her alive but helpless, leading Alan to commit a brutal murder by stabbing him to death. The chaos escalates as Sarah and Paula await Alan at his practice. Alan encounters Paula first and, overwhelmed by his spiraling mental state, viciously drills into her tooth, destroying it, before snapping out of his trance. When Jessica (his assistant) questions his behavior, Alan fires her, and she later confronts him with evidence of his crimes—prompting him to kill her as well to keep her silent.
The investigation intensifies at the police station, where Detective Sunshine and Detective Gibbs find evidence that links Alan to Paula’s dog’s death through a matching bullet. Extorting him using his tax issues, IRS agent Marvin Goldblum tries to secure a dental exam in exchange for silence, but Alan tortures him instead, revealing his lack of remorse. The detectives confront him at his home, discovering the mutilated Brooke and the corpse of Matt. They rush to rescue Sarah, who has managed to escape and hide, just before Alan’s violent attack on her.
Alan’s descent reaches its peak when he arrives at a university, where he teaches dentistry. He coercively instructs his students to pull their patients’ teeth, hallucinating and shooting a student he mistakes for Matt. The police storm the room just as Alan escapes using a hostage. The climax unfolds in an auditorium where Alan, haunted by his hallucinations, sees an opera singer practicing. Mirroring his own obsession with perfection and beauty, the singer transforms into Brooke, taunting him from behind. Overcome by despair, Alan falls to his knees, finally surrendering to the authorities.
The story concludes with Alan in a psychiatric hospital, where his mental state is observed and treated. In a chilling twist, he undergoes another dental procedure, but the person working on him is revealed to be Brooke, now unrecognizably mutilated, working violently on his mouth. This haunting ending leaves viewers with a disturbing glimpse into the cycle of his obsession and mental illness.
