Dr. Alan Feinstone has everything. A beautiful wife, a nice house, a job as a dentist, a popular reputation and is obsessed with order and cleanliness. However, everything changes, on the day of his wedding anniversary; he spies his wife Brooke having sex with their filthy poolman, Matt. Feinstone waits for them to finish, retrieves his handgun (a .380 Walther PPK) and then follows Matt in his car.
He is led to Paula Roberts's house, a friend of Brooke's. Hiding the gun behind his back, he quickly makes up a story about inviting Paula to a surprise party for Brooke. He then sneaks into her backyard in time to see Paula invite Matt inside. Paula's dog, a rottweiler, emerges from the yard and attacks Feinstone. He shoots the dog through the head in self defense and proceeds to drive to work.
At his dental practice, Feinstone's first patient of the day is young Jody Sanders, there for his very first dental appointment. Feinstone begins to clean Jody's teeth. Everything goes smoothly at first, until he imagines that Jody's teeth are brown and rotten. His dental pick slips, stabbing Jody in the gums. Jody's mother picks up her crying, bleeding child and leaves angrily.
As Feinstone sees his second patient, beauty queen April Reign, Detective Gibbs investigates the death of Paula's dog. He suspects the perpetrator was the burglar who has been robbing houses in the neighborhood, though that criminal has never used a gun before.
Alone with April, Feinstone sedates her with nitrous oxide so that he can fill a cavity in one of her molars. As she drifts off into unconsciousness, Feinstone imagines that she has transformed into his wife. He begins kissing and fondling his "wife" on the dental chair, then begins to choke her. April starts to cough and half-wakes up from the gas. Feinstone snaps out of his trance and quickly re-buttons April's blouse and finishes removing her pantyhose. Hiding the pantyhose behind a counter, Feinstone helps the still-coughing April into the arms of her manager, Steve Landers (Mark Ruffalo). He leads them out claiming April is dizzy from the nitrous oxide. Steve returns a few minutes later and furiously punches Feinstone hard in the mouth presumably after what April told Steve about her encounter with Feinstone. They leave angrily, threatening a lawsuit for sexual and physical assault on their way out. Feinstone decides to end the day early and sends his staff and patients home. Especially disappointed is Sarah, a teenager who desperately wants to get her braces removed. Candy, Feinstone's friendly office manager, tells her that she'll have to wait until tomorrow.
Later that night, Brooke meets Feinstone at his practice. He reveals his new Italian opera-themed patient room. He encourages Brooke to try out the room's dental chair and clean her teeth. When she does, Feinstone binds her to the chair with a scarf and sedates her with nitrous oxide. With operatic music blaring in the background, he begins to pull out Brooke's teeth and cuts out her tongue.
Investigating the burglaries, Detective Gibbs and his partner Detective Sunshine arrive at Feinstone's door the next morning to ask him some questions. As they speak, Brookestill alive, but sedatedlies on a pool chair with a bikini on to distract Matt in the backyard while Matt cleans the pool with a scoop. Just as the policemen leave, Matt finds what seems to be a severed tongue in his net. He lifts Brooke's hat from her face to alert her of this discovery. She moans, reaches for him and displays her mutilated mouth; all her teeth are missing and her tongue is completely cut. Feinstone emerges from the house with a kitchen knife and stabs Matt to death.
Waiting for Feinstone when he arrives at his practice are Sarah and Paula. Feinstone sees Paula first, much to Sarah's disappointment. When Paula's conversation turns to how good a job Matt does for her, Feinstone asks for a dental drill and begins overly-aggressively drilling her tooth. As her tooth is destroyed, his assistant, Jessica, questions what he is doing. Feinstone snaps out of his trance and asks Jessica to finish up. Instead, Jessica whispers to Paula to leave. When Feinstone discovers that Paula has left, he fires Jessica on the spot. She pulls out April's pantyhose from the previous day and threatens to expose Feinstone. Grabbing the hose from her hands, Feinstone wraps them around Jessica's neck and strangles her to death.
At the police station, Detective Sunshine discovers that the bullet pulled from Paula's dog's murder scene only matches one gun in the area: the Walther PPK owned by Feinstone. He and Detective Gibbs drive to the Feinstone house to question him further. Near the pool, they discover Matt's body. They quickly break into the house and find the mutilated Brooke, tied to the bed but still alive.
Meanwhile, IRS agent Marvin Goldblum, using Feinstone's tax problems as leverage, extorts a free dental exam and tries to extort a payoff from the doctor. This does not go over well with Feinstone. He inserts a retractor into Marvin's mouth and cracks his jaw wide open. He then uses a dental drill to slice into Marvin's tongue. Later, Feinstone's other assistant, Karen, finds Marvin still in the dental chair. The doctor grabs her and inserts a needle full of air into her jugular. The air bubble travels into her brain and kills her.
As the flirtatious Candy leaves for lunch with dental equipment salesman Matthew Zeigler, Sarah finally gets called back to have her braces removed. Feinstone removes them and shows Sarah her smile in the mirror. As with Jody, he imagines her teeth brown and rotting. He pulls out his gun and aims it at her forehead. Sarah slams the exam light into Feinstone's head and escapes. She hides in one of the dental rooms, where Sarah finds the blood-soaked Marvin, who attacks Feinstone, but Feinstone recaptures her. Hysterical, Sarah promises to brush her teeth three times a day and to never eat candy. This satisfies the doctor and he leaves. The two detectives arrive, but they are a little too late.
They follow him to a university where the doctor teaches dentistry classes. There, Feinstone at gunpoint manically instructs all of his students to pull all of the teeth out of all their patients. His vision warps again and he sees all of the people he has mostly killed or mutilated over the past few days, including Matt. He shoots Matt, who turns out to be a dental student. The detectives burst into the room, but Feinstone uses a hostage to escape. Eventually, he wanders into an auditorium where an opera singer is practicing. Enchanted, he watches her from behind. When he reaches out to touch her, she transforms into Brooke, who laughs at Feinstone. Defeated, he gets on his knees and is arrested by the detectives.
In the final scene, we see Feinstone in a psychiatric hospital. He is carted off to his regular dental appointment, when his toothless wife Brooke begins to work violently on his mouth.