The novel begins with an "oath" signed by John Conlan and Lorraine Jensen, two high school sophomores, who pledge that they will report only the facts about their experiences with Mr. Pignati.
When John, Lorraine, and two teen troublemakers, Norton Kelly and Dennis Kobin, are bored, they make prank phone calls. The goal of the game is to see who can stay on the phone the longest. When it is Lorraine's turn, she picks out Mr. Pignati's phone number and pretends to be calling from a charity. After winning the game with a long conversation with him, Mr. Pignati offers to donate ten dollars. Against Lorraine's better judgement, John and her travel to his house to collect the funds. After hesitantly accepting "The Pigman's" offer of going to the zoo, a friendship begins to blossoms between the three of them. He begins to take on the role of a parental figure for the two teenagers, something neither of them have.
John and Lorraine's visits become increasingly frequent, and during one such visit, they discover a document inside his room. After reading it, they realize The Pigman has been lying this entire time about where his wife has been. His wife, Conchetta, is dead, instead of being on vacation as The Pigman has stated numerous times. Soon, John and Lorraine visit The Pigman daily after school, and he showers them with gifts, food, and most importantly, the love and attention they do not receive in their own joyless homes. They reveal to him that they were never affiliated with any charity, and he reveals what they already know: that his wife is dead.
One of the most important gifts the kids receive are a pair of roller skates. Getting a pair for himself also, the three of them could not be happier, until one afternoon. Mr. Pignati suffers a heart attack while he and the teens are playing tag with roller skates, and is sent to the hospital; John and Lorraine agree to take care of his house while he recovers. While they are doing so, they resemble a married couple. Between the responsibilities and numerous chores, they love being inside the house. They both begin to even acquire feelings for one another, and John begins to care about his appearance. Having an empty house to themselves, the kids decide to have a party and invite a few people over. The true betrayal comes when John invites their so-called friends over to Mr. Pignati's house. The situation quickly turns into a drunken, boisterous party with a lot more people than the two anticipated. Everyone at the party destroys the house and none of them care what they break and whose it is. Lorraine's friend rips Conchetta Pignati's wedding dress in a drunken accident after putting it on. Norton ransacks Mr. Pignati's house in hopes of finding valuables and destroys Conchetta's collection of porcelain pigs, which Mr. Pignati holds very dear to him. John beats him up in retaliation.
Mr. Pignati returns to find his house ransacked, and is incredibly hurt when he finds out John and Lorraine were responsible for the incident. The cops are called as John and Lorraine believe they will get arrested, but The Pigman does not press charges. They try to go back into the house and apologize, but the officer tells them Mr. Pignati is crying and that they need to go home. After going home, Lorraine is beaten by her mother and John's parents say they are getting him therapy, which will never happen. Feeling terrible, the two offer to take him to the zoo to help make up for the destruction of his house. When they arrive at the zoo to meet Bobo, they discover that Bobo the baboon, Mr. Pignati's favorite animal and buddy, has died. Overcome with grief and the heaviness of the recent events, Mr. Pignati suffers from cardiac arrest and dies, leaving John and Lorraine grieving and reflecting on the fragility of life. John tells Lorraine to wait outside of the area where he died, fearing that her mother would hit her in repercussions of the situation. They blame themselves for his death, and believe that he would have been better off never meeting them in the first place. John and Lorraine write their story down.