Writer Jimmy Breslin begins narrating the film; telling the viewer about New York City's current prosperity. However, things were not always this way, showing news articles from 1977 about the .44 Caliber Killer and explaining how the city was in a massive state of decay, explaining that "there are 8 million stories in the naked city, and this is one of them."
We are shown a disgusting apartment with a man screaming to stop an incessant dog barking. Eventually, he goes out and shoots two girls talking in a parked car. The film cuts to Vinny (John Leguizamo) and his wife Dionna (Mira Sorvino) going to a night club for dancing and to meet up with their friends. Dionna's cousin asks to be taken home and Vinny offers to give her a ride, allowing Dionna to stay. We soon discover the real reason he offered to take her home was to have sex with her in the back seat of his car. However, before he can finish, the two are scared off by another couple in the car behind him. As Vinny speeds off, the killer comes out of the bushes and kills the couple.
While driving Dionna home, she begins questioning what took Vinny so long to drive her cousin home. Vinny offers the excuse that he had to urinate but before she can question him further, they arrive at the exact same spot where Vinny was having his affair, only now it is a crime scene. Vinny goes to investigate and to his horror, it is the same couple that scared him off. Vinny begins vomiting as he gets back to the car, telling Dionna that the .44 Caliber Killer is back. The next day as Vinny's friends; Joey T (Michael Rispoli), Brian, and Anthony inquire about the killer, they tell Vinny that the killer probably saw him and will come after him too. Although they are joking, Vinny begins taking this to heart. Soon after, Ritchie (Adrien Brody) shows up in a punk fashion to which all except Vinny object to. Ruby (Jennifer Esposito) is the last to arrive and after being dumped by her boyfriend who was only using her for sex, immediately takes an interest in Ritchie and his new style.
At Dionna's father's restaurant, the local mob boss Luigi (Ben Gazzara) is asked by one of the detectives (Anthony LaPaglia) on the .44 Caliber Killer case for help. Luigi initially refuses, but after hearing the letter left by the .44 Caliber Killer, now called the Son of Sam, agrees. Meanwhile, as Ruby discusses her current predicaments with Vinny at the salon, Ritchie comes in asking for money as he has been kicked out and forced to move into his garage after walking in on his mother and stepfather having sex. Vinny offers to give Ritchie a place to stay for a while, but Ruby convinces him to move into the garage and fix it up with her help. That night as Ritchie and Ruby finish redecorating the garage, Ruby attempts to give Ritchie oral sex but he asks her to stop. Before she can leave, Ritchie explains that just because he doesn't want to have sex with her, doesn't mean he wants her to leave.
Back at the salon, as Vinny and his boss Gloria (Bebe Neuwirth) begin having sex, Vinny begins feeling guilty, but quickly gets over it and has sex with her. After being told by Anthony and Brian that he no longer fits in in the neighborhood, Ritchie takes Ruby to a gay theatre where he has been dancing and secretly selling his body for money. During Ritchie's performance, the owner, Midnite (Michael Imperioli), asks Ruby what she likes to do as she is too hot to be giving it away for free. Outside the theatre, Ruby admits that she has fallen for Ritchie and begins dressing in punk. Meanwhile, after yet another letter to the police from the Son of Sam, Vinny's paranoia reaches a boiling point and although Dionna tries to comfort him, he is unable to perform with her. With the police making no headway with the Son of Sam, Luigi decides to take matters into his own hands and catch the killer and offers another $5,000 to the reward hoping at least to get more names on their list of suspects. Meanwhile at a pizzeria; Joey T, Anthony, Brian, and the heroin addict Woodstock are devising their own list which is primarily made up of people they don't like. At the top of their list, however, is Ritchie.
While at a diner, Vinny and Ritchie discuss their relationships but are asked to leave as Ritchie is scaring some of the other patrons. Before a fight can break out, Ritchie breaks a glass over his head to intimidate the other customers trying to pick a fight with him and Vinny. As they walk down the beach, Ritchie tries to explain himself to Vinny, but the conversation proves to be too deep for him to follow. With temperatures reaching 104, a blackout hits the Five Boroughs. To defend themselves and their neighborhood from the Son of Sam from trying anything, Luigi has a block party for everyone while Joey T leads a mob to patrol the neighborhood, beating and attacking anyone who tries to enter. As this is going on; Ritchie, Vinny, Dionna, and Ruby are having dinner where Ritchie invites them to come see him and his band perform at the club CBGB, while in the bathroom, Dionna asks Ruby for advice on what to do about Vinny as she doesn't seem to please him. The only advice Ruby can give is to not be married to him. Vinny and Dionna go to the club, but are scared off by all the people outside. Instead, they try to get into Studio 54, but the bouncer refuses to let them in. They are however greeted by another man who takes them to another club: Plato's Retreat. There, he and Dionna get stoned and have sex as well as three-ways with other people. On the way home, Vinny mocks an already embarrassed and humiliated Dionna out of jealousy more than anything. In the following fight, Dionna reveals that she knew he had sex with her cousin and drives off, leaving Vinny chasing after the car.
As July 29 arrives, Joey T and the others begin slashing the car tires of everyone on their list, while we see the real Son of Sam descending further into madness and is now seeing a black dog telling him to go out and kill. The next day at the diner, Vinny, Joey T, and the others are eating when the owner come over tells Vinny that he and Ritchie have been kicked out due to Ritchie's actions. Before Joey T can get more information, Vinny storms out. Meanwhile Bobby the Fairy, yet another friend of Joey T's group, sees Ritchie in the gay theatre to which Ritchie threatens to kill him should he tell anyone. Although Ritchie is only trying to protect his name, Bobby takes this literally. That night, we see the Son of Sam kill yet another two victims in Brooklyn.
Joey T and the others are now beginning to suspect that Ritchie is the Son of Sam as he is the only one whose whereabouts they cannot account for, but Vinny stands up for him, saying that he knows Ritchie as if he were his brother. They all reveal to Vinny that Ritchie has been dancing and shooting porn films with Ruby at the gay theatre and ask if he knew that. Although Vinny is shocked by this, he still refuses to accept that Ritchie is the Son of Sam, but reluctantly agrees to help Joey T and the others by telling them where Ritchie and his band perform. Joey T and Anthony go to CBGB to try and find Ritchie while outside, Vinny has already met up with him. He begins to ask Ritchie if hes in a cult, dancing in a gay joint, and is the Son of Sam. Ritchie is upset that Vinny actually would think that way about him, but is angered that he brought Joey T down to get him and walks off. The following day, a composite sketch of the killer is released and everyone sees someone different. No surprise, Joey T tries to convince Vinny that its Ritchie; however Vinny doesn't see any resemblance until Joey T draws in spiky hair on the picture. Looking more like Ritchie now, Vinny begins freaking out. So much so that when he goes to the salon, he explodes on Gloria and dumps his drink on her, storming off. By the time he gets home, Gloria has already called Dionna and told her about all the women he cheated on her with. Vinny attempts to apologies and tries explain that Ritchie may be a killer, but it is too late; a heartbroken Dionna leaves him. This sends Vinny spiraling out of control as he begins drowning himself in cocaine and Quaaludes.
With Richie back at his parents' garage, Joey T and the others decide to make their move and capture Ritchie and take him to Luigi. However before they go, Joey T decides to get Vinny. They break into Vinny's house to find him stoned and force him to come help lure Ritchie out of the house, promising not to hurt him. Unbeknownst to them, during this time, the police have captured David Berkowitz, the real Son of Sam. Meanwhile, Vinny comes knocking on Ritchie's door and tells him that Dionna left him. Ritchie invites him in, but Vinny asks him to come outside, not wanting Ruby to hear. As the two talk, Vinny tries to tell Ritchie to run, but before he can understand what Vinny is saying, Joey T and the others spring on him. Vinny watches as Ritchie uses his guitar to defend himself, but is quickly beaten senseless by the group. Ruby is restrained by Bobby as they drag Ritchie off. However, they don't get too far as Ritchie's stepfather, Eddie (Mike Starr), comes out of the house, brandishing a Luger pistol. As Joey T tries to explain who Ritchie is, Eddie reveals to them that the cops had already caught the Son of Sam in Yonkers, much to the group's surprise. As they begin to leave, Vinny can only look at a bloodied and beaten Ritchie with guilt and weakly attempts to apologize to his former friend before walking away too.
Jimmy Breslin recaps the events of the summer of 1977; David Berkowitz was done in by a parking ticket and is serving six consecutive terms of 25 years to life, the Yankees won the World Series, Elvis Presley died, and 3,700 were arrested during the blackout. He finishes by reiterating that "there are 8 million stories in the naked city, and this was one of them."