During their childhood, John Farley (Seann William Scott) and many of his classmates were tormented by their sadistic gym teacher, Mr. Jasper Woodcock (Billy Bob Thornton). Since then, things have turned out quite well for John, who has gone on to become a successful self-help author and motivational speaker about "letting go", inspired by his own experiences. But when John's hometown offers him an award, he decides to return home and face his unpleasant memories. His manager initially rejects it as too trivial without consulting Farley, but Farley is pleased by the award and returns to Nebraska to receive it.
When John returns home he is horrified to find that Woodcock is now dating his widowed mother Beverly (Susan Sarandon), and the two become engaged shortly after. To make matters worse, Mr. Woodcock is being awarded for his years of educating children. Mr. Woodcock's behavior toward Farley is still unpleasant. John tries to break up the engagement to save both himself and his mother.
To that end, he decides to beat Mr. Woodcock at his own game. He spends the whole day with him, going everywhere with him. First, they visit a gym, where, after a rather long speech, Mr. Woodcock berates him for constantly talking. The two then go on treadmills, where John attempts to run faster than Mr. Woodcock, and is launched into a set of weights behind him. They then go into a retirement home where John meets Mr. Woodcock's father, a rude wheelchair-bound man. Mr. Woodcock makes the elderly people in the pool run in place, and take laps. He then spots Pulumbo, a wheelchair-bound man sitting in the corner of the room, not swimming in the pool, and decides to push him in. John, thinking he is unable to swim, tries to dive in and save him, but is knocked out by the man (who apparently can swim), and has to be given mouth-to-mouth resuscitation. When he gets home, his old classmate and that man's younger brother are there (with the boy having a black eye), with fake evidence of Mr. Woodcock's abuse, consisting of the boy begging an off-camera man he claims to be Mr. Woodcock not to do something, and is immediately hit in the head with a folded up chair. When Mr. Woodcock and Beverly come home, the fake evidence immediately cuts to a scene of the man driving in his truck, ranting about how hot Farley's mom is, wishing he could motorboat her breasts. He tries to rewind the tape, but only ends up repeating parts of his rant, for which he's ejected from the house by Mr. Woodcock. At the carnival, Woodcock is better at various games, though John wins in an eating contest, but not before embarrassing himself in front of the crowd.
Irritated, he meets up with his old classmate, who says that he has found Mr. Woodcock's ex-wife, whom Mr. Woodcock's father had mentioned earlier. She tells John that they divorced because of infidelity; the reason she had not done that earlier was the great sex with Mr. Woodcock, once he made her black out from pleasure, and made her speak Portuguese, despite having no knowledge of the language. John, thinking Mr. Woodcock is cheating on his mother, decides to expose it to his mom, and he and his classmate go to Mr. Woodcock's house. As they are trying to find evidence, Mr. Woodcock comes up the stairs, forcing them to hide. As soon as Mr. Woodcock comes in, John's mom comes in as well, and they have sex on the bed (making her speak a phrase in Portuguese in the proccess) as John suffers underneath them. With excitement, his friend opens the shutters and records the whole scene.
Next they are at the award ceremony for Woodcock. One by one people come up to the stage and praise him for the good that he has done in their life. Then John is awarded the "Corn Cob Key" and during his acceptance speech, he sums up the misery Woodcock has supposedly caused, but he makes a fool of himself. Much of his evidence is busted, as his classmate did not die as he thought and his ex-wife cheated on him, instead of visa-versa.
After Woodcock threatens to beat up John, Beverly becomes so mad that she breaks up with Woodcock. The next day, John has to do a live video interview with Tyra Banks. His mother tells John that he never thought of what made her happy and always thought about himself. John runs out of his interview in the middle of it and apologizes to Woodcock. They then get into a fight that turns into them, in full costumes, wrestling. It ends with Jasper getting rushed to the hospital by John. John rushes Jasper on his gurney to his mom who is sitting atop a float on a parade. After his gurney flips over on the "one pothole in the town" his mother takes him to the hospital where John gives his blessings to his mother and tells his mother that Jasper is a very good person and molded him into what he is today.
The film ends at christmas time with Jasper telling John that he read his book. Says one good thing about it, then burns it. After his next book is released, Jasper tells John that his mom is pregnant and the film closes with him saying "Just joking".