Two high school sophomores in New York City, John Conlan and Lorraine Jensen, are writing a record of their experiences with a man named Angelo Pignati, whom they have nicknamed “the Pigman.” John and Lorraine are alienated from their parents, who have no ability to relate to their kids. John and Lorraine are close friends and occasionally hang out with a few misfits, Dennis and Norton, often drinking at the cemetery. Early on, the story provides hints of who the titular Pigman is and why he matters to John and Lorraine; his death has significance to them. As John and Lorraine continue their story, they reveal that Mr. Pignati was their friend and they are deeply saddened by his death but, at the same time, possibly responsible for it.

John, Lorraine, Dennis, and Norton like to make prank phone calls. One day Lorraine calls Mr. Pignati pretending to be with a charity. He offers a donation, and John and Lorraine go to his house after school to collect the money. In his late fifties, Mr. Pignati serves them wine and shows them his pig figurine collection, which he collected with his wife, Conchetta, who is out of town. As Mr. Pignati hands them a check, he invites them to accompany him to the zoo. Initially, Lorraine refuses because she feels guilty that they took his money. But after an unpleasant encounter with her mother, she agrees, and John and Lorraine skip school the next day. At the zoo, they meet Mr. Pignati’s best friend, a baboon named Bobo, and eat lots of treats. The next day they bump into Dennis and Norton, who ask about their new friend. Later that day, John and Lorraine don’t go to Mr. Pignati’s. Instead, John returns home, where he suffers through a typically unpleasant dinner with his parents. After John argues with his father over his future, his mother asks him to leave for the night, so John and Lorraine go to Mr. Pignati’s house, where they are warmly welcomed. On Saturday, they accompany Mr. Pignati into Manhattan for a department store shopping spree. They buy gourmet foods, and Mr. Pignati spoils them with gifts, including roller skates for all of them. In the pet department, they see three monkeys huddled together in a cage, and Lorraine immediately links the monkeys with the three of them, three lonely creatures desperate for love and connection.

John and Lorraine continue visiting Mr. Pignati almost every day. One dark cloud that hangs over their newfound friendship is Norton, who seems interested in learning more about Mr. Pignati and anything valuable he might own. John tries to throw Norton off the scent and vows to protect the older man. One evening, a downcast Mr. Pignati tells John and Lorraine that Bobo wouldn’t eat any of the treats. Mr. Pignati’s obvious dejection pushes Lorraine to insist that she and John finally tell him the truth that they are high school students and not charity workers. Their confession spurs Mr. Pignati to tell them that Conchetta is dead, a fact John had already discovered when he found a funeral receipt in a drawer. To try to lighten the sad mood, Mr. Pignati invites John and Lorraine to play a game. The game consists of Mr. Pignati telling a psychological story involving a number of characters and a murder. After John and Lorraine choose who is most responsible for the murder, Mr. Pignati tells them that their answer reveals that they are interested in magic. After the game, they start roller skating and playing tag. They exertion proves to be too much for Mr. Pignati, who suffers a heart attack and falls down the stairs.

Mr. Pignati goes to the hospital. The next day, claiming to be his kids, John and Lorraine visit. Mr. Pignati insists that he is fine and asks them to look after his house. That night John and Lorraine make dinner and try on Mr. and Mrs. Pignati’s clothing. They look glamorous, like actors, and John kisses Lorraine. That week at school, things are awkward between the two of them. John and Lorraine also spend less and less time at Mr. Pignati’s house until Friday, the day before he is due home. On Friday, they skip school and spend the day there, bickering like an old married couple and cleaning up. That evening John announces that they will have a few friends over for drinks. While Lorraine is not keen on this plan, she goes along with it, and soon enough the party has grown to include a band and dozens of drunken kids. Norton arrives, even though John did his best to exclude him. John keeps an eye on Norton, certain he is looking for things to steal. Norton disappears, and John discovers him in the act. The two boys scuffle, and Norton takes off downstairs, where he enters the pig room and starts smashing pigs, looking for hidden money. Meanwhile, Lorraine announces that Mr. Pignati has arrived in a taxi. A drunken John can barely process what she is saying and instead launches himself at Norton. The boys skid down the hallway and land in front of Mr. Pignati, who is not smiling. John passes out.

The police arrive and take John and Lorraine home. Lorraine fights with her mother, realizes that her mom is unable to understand her, and cries herself to sleep. John’s father tells him he must go to a psychiatrist. The next day, John and Lorraine meet up and persuade Mr. Pignati to come with them to the zoo to visit Bobo. However, the baboon has died, and Mr. Pignati, shocked and lost, has a heart attack on the spot and dies. John sends Lorraine away so she won’t get in trouble with her mother. He waits with Mr. Pignati while the ambulance comes. Existential thoughts run through his head about growing old alone, about death and decay, and about turning into people like his parents. For a moment, he feels angry at Mr. Pignati for his own death, blaming him for befriending kids. But he also blames himself and Lorraine for pretending to be grown up. They all have been punished for trespassing into a realm where they don’t belong. Now, John and Lorraine finally realize that they are wholly responsible for their own futures.