Plot

Soon-to-retire Detective Lieutenant William Somerset (Morgan Freeman) is partnered with short-tempered but idealistic Detective David Mills (Brad Pitt), who has recently moved to an unnamed large city with his wife Tracy. Somerset has 7 days to go for his retirement. After forming a friendship with Somerset, Tracy (Gwyneth Paltrow) confides to him (after she invites Somerset for dinner) that she is pregnant and has yet to tell Mills, as she is unhappy with the city and feels it is no place to raise a child. Somerset sympathizes, having had a similar situation with his ex-girlfriend many years earlier, and advises her to tell Mills only if she plans to keep the child.

Somerset and Mills investigate a set of murders inspired by the seven deadly sins: a morbidly obese man (Bob Mack) forced to eat until his stomach burst, representing gluttony (Somerset figures that the detail contained in the murder signifies that the act of death means something), and a criminal defense attorney (Gene Borkan) killed after being forced to cut a pound of flesh from himself, representing greed.

Clues at the murder scenes (The picture in the lawyer's office was hanging upside down and behind it they find fingerprints that don't belong to the victim) lead them to a suspect's apartment, where they find a third victim (Turns out the fingerprints were from a finger that the killer cut from the 3rd victim), a drug dealer (Michael Reid MacKay) and child molester, strapped to a bed, emaciated and barely alive, representing sloth. The third victim is in critical condition and unable to respond to Somerset and Mills' questioning. Daily photographs of the victim, taken over a year, show the crimes were planned far in advance.

The detectives use library records (Mills had blurted earlier that the suspect might have a library card) (Somerset prints a list of all books related to the 7 deadly sins and uses his contacts at the FBI to find out who has been reading them) to identify a John Doe (Kevin Spacey) and track him to his apartment. Doe flees and Mills gives chase, during which Mills falls from a fire escape and injures his arm.

Mills searches a truck before being struck in the head with a tire iron. While Mills is incapacitated, Doe walks up and holds him at gunpoint for a moment before escaping. The apartment contains hundreds of notebooks revealing Doe's psychopathy, as well as a clue to another murder. The detectives arrive too late to stop a man forced by Doe at gunpoint to kill a prostitute (Cat Mueller) by sexually assaulting her with a custom-made, bladed strap-on, representing lust. The following day, they attend the scene of a fifth victim, a model (Heidi Schanz) whose face has been mutilated by Doe; she was given the option to call for help and live disfigured or commit suicide by taking pills, and chose the latter option, representing pride.

As Somerset and Mills return to the police station, Doe unexpectedly turns himself in, covered in the blood of an unidentified victim. Doe offers to confess to his crimes, but only on the condition that he escort Somerset and Mills to a secret location where the victims representing envy and wrath are presumably buried, otherwise he will plead insanity. Somerset is wary, but Mills agrees. During the drive, Doe expresses his lack of remorse for his crimes, declaring that his victims deserved to die, and professes himself to be a martyr chosen by a higher power to shock the world out of its state of apathy. Doe also makes cryptic, threatening remarks towards Mills, who maintains his belief that Doe is simply insane.

Doe's directions lead the detectives to a remote deserted location. Minutes after their arrival, a delivery van approaches. Mills holds Doe at gunpoint while Somerset goes to intercept the driver, who says he was instructed to deliver a box to their coordinates. Somerset opens the box and, in a sudden panic, tells Mills to stay back. Doe reveals that he himself represents the sin of envy, as he envied Mills' life with Tracy and implies that the box contains Tracy's decapitated head. He goads Mills, telling him that Tracy begged for her life and the life of her unborn child, and reacts with surprise and delight when he realizes Mills was unaware that Tracy was pregnant.

Despite Somerset's warnings, an enraged Mills fulfills his own role as wrath and shoots Doe fatally and repeatedly, completing Doe's plan. Somerset and the police captain (R. Lee Ermey) watch as the devastated Mills is taken away. When the captain asks where he'll be, Somerset says he will be around, implying that he will not retire.

In a voiceover, he then quotes Hemingway: "Ernest Hemingway once wrote 'The world is a fine place and worth fighting for.' I agree with the second part."