Detective Lt. William Somerset (Freeman) (possibly named for W. Somerset MaughamSe7en is preparing to retire from police work after many gruelling and unpleasant years of dealing with the destitution and apathy bred within the city (the city's identity is left open throughout the film), which is depicted as sordid and dark. In his last week, he is partnered with Detective David Mills (Pitt), a much younger and more naive officer who just relocated to the department from elsewhere in the state.

They meet one another at a crime scene in which an obese man who was force-fed, bound and tortured, lies dead. He has wires on ankles and wrists, and there is a bucket of vomit under the table. A pathologist later verifies that the man was fed repeatedly, then kicked in the side so he burst. This caused his stomach to split and led to an internal hemorrhage that brought on his demise. The first bit of evidence that has the two detectives believe they are after a killer with a grudge is Somerset's discovery of two shopping receipts, indicating that the killer had left the cockroach-infested, filthy apartment to visit a supermarket in between force-feeding the victim, who had eaten all the food in the house.

After their captain (R. Lee Ermey) confronts the detectives in his office, Somerset argues that Mills should be placed on a different assignment. However, soon after, the gruesome murder of the prominent Jewish lawyer Eli Gould is discovered. Gould was made to excise a pound of his own flesh in the tradition of William Shakespeare's Jewish character Shylock. A note is left in Gould office with an extract from , and written on the floor in Gould's blood is the word GREED. Somerset goes back to the first victim's house and does some re-investigating, and finds GLUTTONY written in grease behind the refrigerator of the apartment in which the obese man was murdered. He also finds a note reading "Long is the way and hard that out of Hell leads up to light", a quote from Milton's Paradise Lost. He begins to suspect that the crimes are related, and confronts his superior to warn that there will be five more murders, each patterned after the remaining five of the seven deadly sins. He says that the Milton quote means the murder spree is just beginning.

Somerset and Mills team up once again, and all previous tension seems to be obliterated after Mills' wife Tracy invites the demoralised and pessimistic detective to their new house for dinner. That same evening, they find a set of fingerprints at the site of Gould's murder. The evidence, hidden behind a painting which Mrs. Gould notices has been turned upside down, belong to a known child molester and drug dealer, but as the task force prepare to storm the offender's residence the following morning, Somerset is already sure that he is not the person they are looking for.

He is proven right when the man is found tied to his bed, alive but suffering from severe muscular deterioration after having spent a year completely immobile, Somerset once again voices concern that they stand little chance of catching the cold-blooded, calculating killer, who photographed the process of the tied man's deterioration and manipulated the evidence the detectives collected to ensure that they discovered his victim exactly one year after he rendered him immobile. Besides the fact that the victim's brain is completely 'mush' and he has "chewed off his own tongue", the perpetrator severed his hand, which explains how his prints turned up at the scene of Eli Gould's murder. The word SLOTH is written on the wall.

Later Somerset receives a phone call from Mills' wife Tracy requesting a private talk. They meet, and she informs Somerset that she is pregnant, but is not sure bringing a child into this world would be wise. She tearfully says that she hates the city. Somerset tells her he was faced with the same decision at one point in life and chose the path of abortion. He has regretted it ever since, and advises her to never let Mills know she was pregnant if she chooses not to have it. He tells her that if she keeps the baby she should spoil it every chance she gets, and this seems to cheer her up slightly.

With the investigation going nowhere, Somerset pays a contact in the FBI to print out the list of names on the government database of "flagged" library books (e.g. ). Through the list (which is fictitious), they come up with a list of possible matches, one of which is an individual named Jonathan Doe (a play on the John Doe name used for anonymous perpetrators). When they visit Doe's apartment, he opens fire at them from further down the hall and leads them on a lengthy chase through labyrinth-like decrepit tenement buildings. These scenes were shot on location at the run-down yet historically significant Alexandria Hotel in downtown Los Angeles. During the pursuit, Mills is injured by Doe who spares his life but escapes capture. This injury was added to the story after Brad Pitt injured himself attempting a stunt in the scene.

The inside of the man's apartment contains a darkroom and meticulously kept logs of the killer's random thoughts. Amongst the heap that suggests Jonathan Doe is an obsessive maniac, evidence of possible future victims arises. One of them seems to be a prostitute. There is also a receipt from a custom fetish shop for an item that winds up being used in the murder of said prostitute; LUST is scratched into the door outside a murder scene where a frantic man was forced to wear a custom-made leather belt with a strap-on knife and copulate with a bound working girl. Mills and Somerset later argue in a bar about the value of what they are doing, and Somerset is not convinced that staying on as a policeman would make any difference.

A fifth victim turns up the next day after a phone call from John Doe to his own apartment. A model is found dead in her own bedroom. Doe had sliced up her face and cut off her nose "to spite her face," and then bandaged up the wounds. He had then superglued a phone handset to one hand and a bottle of sleeping pills to the other, offering her a choice of either calling for help and living with her disfigurement or suicide by overdosing on the pills. By choosing suicide, she accedes to the sin of PRIDE, which is written in blood on the headboard of the bed.

When Mills and Somerset return to police headquarters, John Doe (Kevin Spacey) confronts them. His shirt is covered in blood. Doe offers to plead guilty but only if allowed to escort the detectives to his final revelation. On the way there, he extensively alludes to the greatness of his achievement, and seems particularly preoccupied with Detective Mills. He offers reasons as to why he has committed the heinous murders, and claims that in order to arouse a ened consciousness in the desensitised, amoral people of today, one cannot expect to tap them on the shoulder and have them listen, but rather hit them with a sledge hammer. He says that these days, people see deadly sins everywhere and just ignore them, but they won't do that anymore. Detective Mills tells him he's not a Messiah, just a "Movie of the Week" or a T-shirt slogan.

When they arrive at Doe's prearranged location, dry and desert-like, a delivery truck pulls up. Inside is a delivery for Mills, which Somerset opens. He tells Mills, who is struggling to ignore Jonathan Doe's comments, not to come near the box. Doe's incessant rambling reveals that he had visited Tracy after Mills left for work and tried to play husband. The independently wealthy Doe envied the fruits of a common man's life and is thus guilty of ENVY. Doe then discloses that he killed Tracy and her unborn baby, and it becomes apparent that Mills was unaware of her pregnancy and that the object in the box is Tracy's severed head. Enraged, Mills pulls out his gun and dramatically contemplates killing Doe. Somerset tries to stop him, arguing that Doe's revelations only stand if he is killed for his sin of Envy and if Mills is the one who kills him and so becomes the embodiment of WRATH. "If you kill him, he wins," says Somerset. The distraught and emotional Mills is overcome by the death of his wife and their unborn baby, whom he did not know existed until Doe told him. He shoots the killer in the head. John Doe drops dead, victorious. Mills shoots him a few more times even though he is already dead.

In the final scene, Mills is taken into custody. Somerset is assured by the captain that, for the immediate future (Mills's final fate is not known), Mills will be taken care of. The film concludes when, in voice-over, sirens wailing in the background, Somerset says "Ernest Hemingway once wrote: the world is a fine place, and worth fighting for. I agree with the second part."

The gruesome photographs and flashback scenes all appear in black and white. Fincher uses cliché visual elements such as heavy rain and an overcrowded and polluted city along with dark skies to establish a sense of a soiled society. When detective Somerset in getting into a cab late on a night when it is raining mercilessly right after he discovers that he is dealing with a serial killer and the cab driver asks, "Where are you headed?" Somerset replies "far away from here" as he gazes out the window. Fincher also uses film noir standards in the costumes of his characters. Detective Mills and Somerset are wearing old fashion trench coats (to shield them from the constant rain), and dark and bleak suits.

Morgan Freeman plays the role of William Somerset, a detective Lieutenant who is nearing his retirement. Somerset is an elderly man who has, over many years doing detective work in the crime filled city, become very prudent and wise. Somerset’s wisdom, and general all-knowing air, is seen as he predicts the nature of the crimes and their origin. His prudence is seen at the beginning of the movie, while he is getting dressed. His clothes are ironed, his pen, pocket knife, and wallet are formally placed on his dresser, he is clean shaven, and pays very close attention to his appearance. At the beginning of the film Somerset has a gloomy attitude about the world. He sees the world as being an unconscious and heartless place on which he is almost ready to give up. Through interactions with detective Mills, his attitude is changed and by the end of the movie Somerset’s all-knowing gloomy attitude has replaced by a more cautiously optimistic view. Somerset says at the end of the movie that the world is a place worth fighting for.

Brad Pitt plays the role of detective David Mills a upcoming detective who is arrogant and boastful. He is married to Tracy Mills, Gwyneth Paltrow, and has two dogs. Mills will be taking the position of detective lieutenant after detective lieutenant Somerset retires. Detective Mills is the complete opposite of detective lieutenant Somerset. Mills's constant bragging, short temperedness, and impatience attest to his inexperience and lack of wisdom. Mills has an optimistic and tenacious attitude on life. He is willing and ready to take on the horrid case. At the end of the film Mills lack of prudence is seen as he falls victim to the dreadful plan of John Doe.

Kevin Spacey plays the role of John Doe, a serial killer who is obsessed with the seven deadly sins. Doe plays a God like character who knows everything and is in control of everything that he desires. His background in the film hint at a possible career as a former reporter, and is drawn into his accumulated life's work with his wealth and time. Doe, like Somerset, views the world as a depraved and demoralized place. Doe wanted to preach to the world about the seven deadly sins, but felt that he could not reach people verbally, so he committed the murders, using them as a method of preaching. At the end of the film Doe says that only in a country like this could someone commit one of these seven deadly sins and be considered innocent by the people around him. Doe says that “you can’t just tap someone on the shoulder and expect them to listen to you; you have to hit them in the face with a sledgehammer”. Doe also justifies the killings by sayings that he was chosen by a higher power to preach to the world about the deadly sins and to do so by committing the gruesome crimes.

Gwyneth Paltrow plays the role of Tracy Mills, the quiet supportive wife of detective David Mills. Tracy acts as the mediator of detective Mills and detective Somerset. When Detective Somerset and detective Mills first met, their interactions are characterized by uneasiness and agitation. After a dinner, planned by Tracy, the two men become more cooperative and less agitated around one another. Tracy’s support for her husband is seen a conversation that she has with detective Somerset. In this conversation she says that she hates the city, but does not say anything because her husband likes it.

The opening credit music was the nearly-unrecognizable Coil remix of "Closer" by Nine Inch Nails, available as