It is the summer of 1977, and New York City lives in fear of the ".44 Caliber Killer", who shoots young women and their male companions. The killer, David Berkowitz, later identifies himself as "Son of Sam" in a note left at a murder scene. Berkowitz lives in a messy apartment, where he is driven crazy by the barking of a neighbor's large black dog and has a vision of the dog directing him to kill.
In an Italian-American neighborhood in the Bronx, hairdresser Vinny and his pretty wife Dionna go disco dancing at a local nightclub, where they meet Dionna's attractive cousin Chiara. Vinny offers to drive Chiara home while Dionna remains at the club. Vinny and Chiara then park on a residential street and have sex in the car. Son of Sam watches them, but he is scared off when the couple accidentally set off the car horn during sex and, embarrassed, quickly drive away. After they leave, Son of Sam kills another couple who had parked behind Vinny. When Vinny picks up Dionna back at the club, she notices the smell of vaginal lubrication on his face and realizes he had sex with Chiara, but does not let on that she knows.
On the drive home, Vinny notices police near the location where he had parked with Chiara, and sees the dead bodies of the slain couple. The religious and guilty Vinny, realizing he could have been a victim, decides that God spared him in order to give him a chance to reform his ways and stop cheating on his wife. Although Vinny loves Dionna, their sex life is suffering because Vinny enjoys anal sex, "69" and other sex acts that he considers kinky, but he cannot bring himself to discuss or perform these acts with a wife. He is also having an affair with Gloria, the owner of the hair salon where he works.
The next day, while Vinny is hanging out with neighborhood drug dealer Joey T and his friends, Vinny's old friend Ritchie, who has been away for some time, suddenly reappears, sporting a punk spiked hairdo and clothing and affecting a British accent. Vinny, Joey T, and the others dislike the change in Ritchie, and he soon finds himself unwelcome in the neighborhood. However, Ruby, a promiscuous local girl, is attracted to Ritchie, and the two begin a relationship. Unlike other men, Ritchie takes an interest in Ruby as a person, not just as a sexual outlet. She learns that he makes money by erotic dancing and prostituting himself at a gay theater, but remains loyal to him and over time begins to dress in punk fashion herself.
As the Son of Sam killings continue, tension rises in the neighborhood. A police detective from the neighborhood asks the local mob boss to help him find the killer; Joey T and his friends also make a list of possible suspects, including Ritchie whom they regard as "a freak". Ritchie and Ruby invite Vinny and Dionna to come see their punk band perform at CBGB, but once there, Dionna feels intimidated by the punk crowd and refuses to go in. Vinny and Dionna instead go to Studio 54, where they are denied entry, and finally end up at Plato's Retreat where they take drugs and participate in an orgy. Vinny becomes upset when he sees Dionna appearing to enjoy the experience of having sex with another man, even though he himself is having sex with other women. He berates Dionna in the car on the way home, causing her to get mad and reveal that she knows he cheated on her with Chiara. She storms off to stay at her father's house. Vinny begins to drink and use drugs heavily, and makes a scene at Gloria's hair salon, causing her to angrily throw him out and then inform Dionna about their affair. Upon hearing from Gloria, Dionna leaves Vinny for good.
Joey T and his gang decide that the latest eyewitness sketch of Son of Sam released by the police resembles Ritchie, and unsuccessfully attempt to track him down at CBGB. Joey persuades the unstable Vinny, who is high on drugs he has taken to dull the pain of his impending divorce, to help them lure Ritchie out of his house, since Vinny is the only local friend Ritchie still trusts. Unbeknownst to Vinny and his friends, the police have already arrested David Berkowitz, the real Son of Sam. Vinny goes to Ritchie's family home, where Ritchie and Ruby are packing up to leave town, and lures Ritchie out on the pretext of talking about his failing marriage; once he has gotten Ritchie outside, he warns Ritchie under his breath to run, but Ritchie does not heed the warning and is attacked and severely beaten by Joey T and his gang. Ritchie's stepfather, Eddie, emerges from the house brandishing his gun and rescues the badly injured Ritchie, telling the attackers that Ritchie is not the Son of Sam and that the TV news is reporting that the police have just arrested the real killer. Unable to face Ritchie, Vinny walks away.
The role of Dionna was originally written with Jennifer Esposito in mind. The role of Ruby was originally offered to Sarah Michelle Gellar. However, a cast reshuffle ended with Mira Sorvino as Dionna and Esposito as Ruby. Journalist Jimmy Breslin, to whom the real Son of Sam sent letters during the time of the murders, appears as himself introducing and closing the film. Phil Rizzuto appears in the film as the Yankees' broadcaster, and boxer Evander Holyfield makes a brief appearance as a man in a riot.
The film was largely shot during the summer of 1998 and set in the Italian-American neighborhoods of Country Club, Morris Park and Throggs Neck sections of the Bronx, with some scenes filmed in Brownsville, Brooklyn. Although most of the Son of Sam murders actually took place in Queens, the double shooting that Vinny narrowly escapes has been called an accurate depiction of the April 1977 killing of Alexander Esau and Valentina Suriani in the Bronx. Marie's Beauty Lounge, the salon where Vinny works, was a real active salon that was located on Morris Park Avenue at the time, between Williamsbridge Road and Bronxdale Avenue. The real CBGB club was also used; the band L.E.S. Stitches shown playing there was a contemporary punk band operating out of New York's Lower East Side.
Adrien Brody's nose was broken during the final climactic fight scene in which his character Ritchie is brutally beaten by his friends. The sex orgy scene at Plato's Retreat included more explicit shots in the original cut, but was edited after the MPAA threatened the film with an "NC-17" rating.
Reviews of Summer of Sam were mixed. Kenneth Turan of the Los Angeles Times stated "Lee is a powerful filmmaker who certainly knows how to have an impact on an audience, but those who survive his ministrations are likely to wonder if in this case the battle was worth the bruises." Todd McCarthy of Variety thought that "this is the closest Lee has yet come to Scorsese territory!"
Summer of Sam has an overall approval rating of 51% on Rotten Tomatoes, with the site's consensus stating: "Spike Lee offers intense visuals but his storytelling feels crowded and overambitious." On Metacritic, the film has a 67/100 rating, indicating "generally favorable reviews".