Generally, this film is a darkly comedic exploration of an "everyman" protagonist whose seemingly mundane and uneventful existence spirals out of control. It all begins when Frank (played by Steve Martin) encounters a gorgeous and enticing new patient named Susan Ivey (portrayed by Helena Bonham Carter), who walks into his dental office with a request for a root canal and some pain relief.
Upon her first visit, Frank promptly schedules her for the procedure the following day and hands her some Ibuprofen for immediate pain alleviation. However, Susan claims to have an allergy to this medication, prompting her to ask for a prescription of the highly addictive painkiller Demerol. Frank, being the cautious professional he is, only approves a prescription for five tablets, but Susan cleverly alters it to fifty when she later picks up her medication.
Complications arise when Susan shows up twelve hours late to her appointment, having completely misjudged the time. She manages to seduce Frank, who ends up getting intoxicated and sleeping with her. In the chaos of the night, Susan steals all of Frank's narcotics, plunging him into a deepening crisis. The next day, a DEA agent appears at Frank's practice, demanding to inspect his narcotic supply after a tragic incident involving a 18-year-old under the influence of cocaine Hydrochloride traced back to Frank’s practice. Knowing that Susan has emptied his supply, Frank fabricates a story, claiming he's dispensed everything to patients, but the agent warns him that he must show the empty containers in two days or face arrest.
That night, Frank confronts Susan at her hotel, insisting she returns the empty containers. Yet, once again, she lures him into bed, and they end up spending the night together. The following day, Frank faces a confrontation from Susan’s brother, Duane Ivey (played by Scott Caan), who threatens Frank with violence, warning him to stay away from his sister: > "Stay the hell away from my sister."
Eventually, when Frank returns to Susan's room, expecting her, he mistakenly finds Duane instead. A struggle ensues, resulting in Frank stabbing Duane in the hand with scissors. Frantically, Frank flees and rushes home just moments before his wife, Jean (whose suspicions are already aroused), arrives. To his horror, he discovers Duane's lifeless body on the floor.
The police soon show up, adding to Frank's turmoil, especially with actor Lance Phelps (portrayed by Kevin Bacon) probing him with increasingly aggressive questions. As the investigation unfolds, it’s revealed that Frank’s teeth marks were found on Duane's body, leading to Frank's arrest for murder. Unbeknownst to him, Jean is the one who actually killed Duane—a result of their own affair and threats.
Once he escapes custody, chaos ensues as Frank learns his brother Harlan has also been killed by Jean. This leads to a bizarre and desperate plan where Frank uses his dental skills to replace his own teeth with Harlan's and sets fire to his dental practice, leaving behind the evidence of a crime scene that will seal Jean’s fate.
In a twist of fate, Frank and Susan, now entangled in a romantic relationship, manage to evade the law and start anew in France, living blissfully in a quaint countryside cottage. Meanwhile, back in the remnants of Frank's once-stable life, the forensic evidence leads authorities to believe he perished in the fire, but Jean is ultimately convicted for what she’s done, finding herself in prison for the crimes that unraveled their lives.
