Soon-to-retire Detective Lieutenant William Somerset is partnered with the short-tempered yet idealistic Detective David Mills, who has just relocated to an unnamed large city with his wife, Tracy. As Somerset is preparing for his retirement in just seven days, a unique bond forms between him and Tracy, who soon confides in him about her pregnancy during a dinner invitation. She expresses her distress about the city, feeling it is not a safe environment for raising a child. Somerset empathizes with her dilemma, reflecting on a similar experience from his past, and advises her to reveal the news to Mills only if she decides to keep the child.
As the investigation unfolds, Somerset and Mills delve into a series of chilling murders inspired by the seven deadly sins. Their first case involves a morbidly obese man who was forced to eat until his stomach burst, representing gluttony. Somerset deduces that these grotesque murder details symbolize something far more profound than simple violence. The second victim is a criminal defense attorney who, in a horrific act representing greed, is forced to cut a pound of flesh from his body.
Clues from the crime scenes lead the detectives to a suspect's apartment, uncovering a third victim — a child molester and drug dealer found strapped to a bed in a state of near death, embodying sloth. This critically injured individual is unable to speak, leaving Somerset and Mills grasping for answers while daily photographs reveal the meticulous planning behind the crimes.
Utilizing library records and some investigative intuition from Mills, they identify a suspect known as John Doe. During an intense chase, Mills falls and injures his arm, only to face Doe who momentarily holds him at gunpoint before fleeing. The suspect's apartment provides a chilling insight into his disturbed mind, filled with notebooks detailing his psychopathic motives alongside a clue to another murder.
In their quest, the detectives are too late to prevent yet another horrific crime where Doe forces a man to murder a prostitute in an act that revels in lust. Soon after, they encounter the fifth victim; her face mutilated by Doe and confronted with a harrowing choice to take her own life or live disfigured, which she chooses — an emblem of pride.
Returning to the station, Doe unexpectedly surrenders, drenched in blood, and insists on confessing his crimes with the stipulation that he will lead Somerset and Mills to the burial sites of victims representing envy and wrath. While Somerset is cautious, Mills is lured into following Doe. During the ominous drive, Doe unabashedly reveals that he feels no remorse for his actions and believes his victims deserved their fates, viewing himself as an agent of justice sent to awaken the apathetic populace.
Eventually, they arrive at a desolate location where a delivery van approaches. Mills, furious and agitated, holds Doe at gunpoint as Somerset approaches the van's driver, who claims he has a package meant for their coordinates. With a mix of dread and urgency, Somerset opens the package, urging Mills to step back upon realizing its contents — the head of Tracy, confirming Doe's sinister implication that he embodies envy due to his desire for Mills' life with her.
In a fit of rage, Mills succumbs to wrath and shoots Doe multiple times, unwittingly completing the twisted plan that Doe crafted. Somerset and the police captain witness the aftermath with a heavy heart, as Mills is led away, broken by grief and remorse.
In a poignant voiceover, Somerset reflects on the impermanence of life and violence through a quote from Ernest Hemingway: > "The world is a fine place and worth fighting for." With deep conviction, Somerset affirms the latter part of the quote, implying his journey is far from over.
