Advertising executive Bill Compton (Patrick), his wife Joan (Audrey Caire), and daughter Melissa (Sarandon) are a wealthy family living in New York's Upper East Side. Melissa has recently been living with her drug dealer boyfriend. After Melissa overdoses and is sent to a hospital, Compton goes to her boyfriend's apartment to get her clothes. Compton then kills the boyfriend in a fit of rage.
Shaken, he grabs a bag of the boyfriend's drugs, flees from the apartment, and goes to calm down in a local bar. There, he hears factory worker Joe Curran (Boyle) ranting about how he hates hippies, and repeatedly saying, "I'd love to kill one." Compton, unable to restrain himself, blurts out "I just did," then fakes a smile once he realizes he has just made a public confession. Joe appears to believe Compton's statement at first, but then takes it as a joke.
A few days later, Joe sees a news report about a drug dealer being murdered a few blocks from the bar. He immediately realizes Compton is the one who did it. Joe, a deeply disturbed man with a hidden violent streak, arranges a meeting with Compton, and the two form a very strange friendship. Compton tells Joe about the phoniness and emptiness of his smug rich friends, while Joe holds Compton in high esteem for doing what Joe could not bring himself to do: kill a rebellious youth. Joe takes Compton to meet Joe's blue-collar friends at a bowling alley, and Compton brings Joe to a bar frequented by advertising executives.
After a very awkward dinner among Joe, Compton, and their wives, Compton tells Joan, who is concerned that Joe might blackmail Compton, that she need not worry because Joe so identifies with Compton that Joe feels as if he were a willing accomplice to the murder. Melissa, having escaped from the hospital and returned to the family apartment, overhears her father confess to the murder. Storming out of the apartment house, she asks her father, "What are you gonna do, kill me too?" Compton tries to restrain her, but she breaks away.
With Melissa missing, Joe and Compton begin to search for her. During their search, they meet a group of hippies at a bar in downtown Manhattan. Joe and Compton tell the hippies they have drugs (the drugs Compton took from Melissa's boyfriend). The hippies invite Joe and Compton to join them at an apartment, where they share their drugs and girlfriends with the pair. They then abscond with the remainder of the drugs, as well as Joe's and Compton's wallets.
When he discovers this theft, Joe beats one of the girls until she tells him that their boyfriends often spend time in an upstate commune. Joe and Compton drive to that commune, and Joe brings along some rifles and plenty of ammunition, "just to scare them." When they see the hippie thieves and demand their wallets back, one of the thieves tosses the pair their now empty wallets and begins to run away. Joe shoots the thief and goes on a rampage, firing at everyone in the commune. Compton protests, but Joe ignores him, mowing down anyone in his path — including people who had no involvement in the theft.
When a new group of hippies arrives at the door and Joe is out of ammo, Joe encourages Compton to join in. Compton shoots and kills several of them, and a girl in the group flees out of the building. Compton runs to the doorway and fires at the fleeing girl's back, hitting her. As she falls, she is revealed to be Melissa. Compton cries out her name and he hears in his mind, once again, her voice asking, "What are you gonna do, kill me too?"