The story is narrated, in alternating chapters, by two high-school sophomores, Lorraine Jenson and John Conlan, who have become friends because of their shared absurd sense of humor and boredom with school. With their loser friends, Norton and Dennis, they prank-call Mr. Pignati and end up going to his house on the pretending to collecting money for the "L. and J. Fund." Mr. Pignati is very welcoming, and invites them to go to the zoo the next day. Mr. Pignati proudly shows Lorraine and John his collection of ceramic pigs, which he and his wife started together when he gave her a ceramic pig as a joke, to remind her of him. Lorraine feels guilty about taking his money, but John is unconcerned, cashes the check, and spends the money on beer and cigarettes.
The next day, Lorraine and John go to the zoo with Mr. Pignati, and meet a baboon whom he calls his best friend, Bobo. After this trip to the zoo, Lorraine and John's friendship with Mr. Pignati deepens, and they give him an affectionate nickname, "the Pigman," though they always address him as "Mr. Pignati."
They begin to go to Mr. Pignati's house nearly every afternoon or evening, and he always makes them feel much more comfortable than they feel in their parents' homes. Mr. Pignati invites them to explore the house, and John and Lorraine learn that Mrs. Pignati is dead when John finds a funeral bill for "Conchetta Pignati." They drink wine, play word games, and go to a department store where Mr. Pignati buys strange gourmet food, such as snails and chocolate-covered ants, which he gleefully shares with them. Mr. Pignati buys all three of them roller skates.
The sinister Norton, who was with John and Lorraine when Lorraine first called Mr. Pignati, becomes increasingly curious about Lorraine and John's friendship with him, and indicates to John that he intends to try to break into the house.
Feeling increasingly guilty for having lied to Mr. Pignati and taken his money, John and Lorraine confess that they are not charity workers. Mr. Pignati, sobbing, tells them his wife is dead. Lorraine and John cheer him up by playing tag with their roller skates on, and Mr. Pignati joins in. He has a heart attack, and goes to the hospital in an ambulance.
Telling the hospital staff that they are his children, Lorraine and John visit him in the hospital. He tells them to make themselves at home in his house during his absence. That night, they dress up in Mr. and Mrs. Pignati's fancy clothes, and John kisses Lorraine for the first time.
The next day, a Friday, John and Lorraine go to Mr. Pignati's house to clean it before he comes home from the hospital the next day. But John decides to invite some friends over for what he says will be a quick drink. But this little get-together quickly turns into a wild party, with about forty of John and Lorraine's classmates. Norton has not been invited, but shows up anyway. John catches him trying to steal an oscilloscope and he and Norton have a vicious fight. John's roller skates and drunkenness prevent him from following Norton into the pig room, where Norton begins to smash the pig collection, looking for money. Mr. Pignati returns home, and sees the wreckage of his house and his pig collection. The police break up the party and take Lorraine and John home. The police tell Lorraine and John that Mr. Pignati is crying upstairs.
The next day, Lorraine and John call Mr. Pignati to apologize and offer to help clean up. Mr. Pignati is listless and quiet. In an effort to cheer him up, they suggest that they all go to the zoo that afternoon. At the zoo, Mr. Pignati is anxious to see Bobo, but he learns that Bobo has died. Mr. Pignati has another heart attack and dies. Guilt-stricken, John and Lorraine resolve to write a "memorial epic" about their friendship with the Pigman.