It is the summer of 1977 and New York City lives in fear of the serial killer Son of Sam who has shot several women in parked cars. One night, Vinny and his wife Dionna go to a night club, where they dance and have drinks with friends. Dionna's attractive cousin wants to leave, which Vinny volunteers to take her cousin home. He takes her to an undisclosed area and has sex with her in the back seat of his car. Moments after Vinny and Dionna's cousin finish and leave the area, the Son of Sam killer arrives and kills two victims that were parked not too far from where Vinny was.
Later on that night, Vinny and his wife drive home, she gives him a kiss and notices a strange taste (deducing that Vinny probably had sex with her cousin, but she was afraid to admit it). While driving with the awkward moment, Vinny drives towards the area that he was at with Dionna's cousin, which is now surrounded by police. Walking to get a better view of what's going on, Vinny sees the dead bodies that the Son of Sam killed right after he left. He returns to his car and begins to throw up, and ignores his wife comforting him to figure out what's wrong.
The next day, as Vinny is meeting with Joey and other guys in the neighborhood in regards to the Son of Sam murder, Vinny is approached by Ritchie, an old friend. Ritchie has become part of the punk rock scene and his unusual dress and hairstyle concerns Vinny and others in the neighborhood. The only one who seems accepting of Ritchie is Ruby, Vinny's half-sister, who thinks he looks good. Ruby is initially shocked when she learns that Ritchie makes money by dancing and prostituting himself at a gay theater but the two become closer and eventually start dating.
As the Son of Sam killings continue, tension rises in the neighborhood. Detective Petrocelli of the New York Police Department asks local mobster Luigi to help find the killer. Luigi and his men begin assembling a list of possible suspects. A group of men in the neighborhood, led by Joey, also make a list of possible suspects, including Ritchie who they regard as "a freak". Later on that day, Vinny and Dionna go to a night club. Afterwards, they have sex.
The next day, Ruby joins a band with Ritchie and they get a gig at CBGB. They invite Vinny and Dionna but the couple are intimidated by the punks at the club and leave. Vinny and Dionna try to get into Studio 54 but are denied and instead end up at Plato's Retreat where they engage in an orgy. On the ride home, an irate Vinny berates his wife for her participation in the orgy, calling her a "lesbian whore." for having a threesome with a man and a woman while Vinny had sex with four other woman during the orgy. This eventually leads to Vinny spitting on Dionna in disgust, to which Dionna slaps him. An argument ensues, with Dionna telling Vinny that she knows about his prior infidelities, including his having sex with her cousin. In the midst of their argument, Dionna eventually drives off and leaves Vinny in the middle of a graveyard. Dionna goes to stay at her parents' house.
After another gig, Ritchie sees Vinny sitting in his car outside CBGB and sneaks up on him, pretends to be Son of Sam and startles Vinny. Their friendship begins to deteriorate as Vinny chastises Ritchie for the recent decisions he's made in his life regarding his appearance and taste of music. Ritchie chastises Vinny for not being a true friend and alluding to the fact that Vinny believes Ritchie may be the Son of Sam. Ritchie curses at Vinny and storms off, with Ruby chasing after Ritchie and advising Vinny to go home.
At this point, Vinny begins to come unhinged (with his marriage failing, the thought of the Son of Sam coming after him, and Joey increasingly trying to convince Vinny that Ritchie is the Son of Sam), increasing his drug use and paranoia. He shows up to work late at the salon and is fired by the owner, Gloria (who he was also having sex with). In a drug-fueled rage, he berates Gloria and gathers his stuff before storming out of the salon. Returning to Vinny's apartment to get her belongings, Dionna prepares to leave an increasingly tired and unstable Vinny. Vinny apologizes to her, promising never to cheat again. However, Dionna doesn't believe him (after Gloria got revenge on Vinny by contacting Dionna and advising her of all of Vinny's infidelities, including Vinny and Gloria's affair). Dionna says she wants a divorce, and after Vinny begs Dionna to stay, Dionna leaves Vinny for good.
The police publish an eyewitness sketch of the murderer and some of the local men think it looks like Ritchie. Vinny, distraught over his impending divorce, gets high on cocaine and Quaaludes. Joey and the neighborhood guys come by Vinny's house asking for his help to lure Ritchie into a trap so they can deliver him to the police. Vinny, high on Quaaludes, begins arguing with one of the men after he embarrasses Vinny about knowing Dionna leaving Vinny and telling the neighborhood. Joey begins to calm the situation down, advising to Vinny that everyone knows Dionna left him and that Vinny's infidelities led to that outcome. Joey threatens to hurt Vinny if he doesn't help Joey and the boys apprehend Ritchie. After being threatened and blackmailed with no other options, Vinny reluctantly goes along with it.
Unknown to Vinny and the men, the police have arrested David Berkowitz for the shootings. Vinny, along with Joey and the boys, goes to Ritchie's place and finds Ritchie and Ruby there. Vinny lures Ritchie out of the house on the pretext of talking about his failing marriage but changes his mind and warns him to run. The other men emerge as Ruby look on horrified, being held back by one of the men. Realizing that Vinny set him up, Ritchie tries to hold the men back with a guitar to no avail as Joey and the guys proceed to viciously beat Ritchie down and drag him towards Joey's car. Ritchie's stepfather, Eddie, comes out of the house and holds the attackers at gunpoint. When Joey alludes that Ritchie is the Son of Sam, Eddie tells them that the killer has already been captured. Eddie, Ruby and Ritchie's mom comfort a battered Ritchie, as someone calls for an ambulance. Unable to face Ritchie because of his betrayal, Vinny walks away with Joey and the rest of the guys.
Who is Vinnie's half sister?
Ruby