John Farley, portrayed by Seann William Scott, is an accomplished self-help author, famous for his bestselling book Letting Go: How to Get Past Your Past. He returns to his hometown in Nebraska to receive the prestigious Corn Cob Key award at the local "cornival". Despite his agent, Maggie Hoffman ([Amy Poehler]), advising him against this trip, believing it to be a waste of time, Farley feels that receiving the award in person is essential.
Upon his arrival, Farley discovers that his widowed mother, Beverly, played by Susan Sarandon, is in a relationship with his former P.E. teacher, Jasper Woodcock, acted by Billy Bob Thornton. Disturbed by this development, Farley recalls Woodcock as a bully from his school days, someone who inflicted pain on the weaker students while relishing in their humiliation. He vividly remembers being taunted by Woodcock, who once berated him, calling him a "fat, gelatinous loser" for not having his gym clothes.
Beverly defends her relationship, stating that Woodcock is the first man she has felt comfortable with since her husband passed away. This does little to quell Farley's anxiety and anger towards Woodcock, especially when he witnesses the two develop a deeper bond. In a desperate effort to protect his mother, Farley attempts to confront Woodcock but is overwhelmed by memories of his painful past, leading him to remorsefully apologize without receiving any acknowledgment of wrong from Woodcock.
As tensions rise, Farley tries to persuade his mother that she deserves better, insisting that Woodcock is not suitable for her. Despite his best friend Nedderman’s ([Ethan Suplee]) encouragement, Farley's obsession with sabotaging Woodcock's relationship with Beverly grows stronger. From dipping Woodcock's whistle in urine to humiliating competitions against him, Farley's antics escalate, including a disastrous encounter at Hal the Barber that results in an unintentional and comical haircut.
Meanwhile, Farley learns about Woodcock's troubled past and makes it his mission to uncover whether Woodcock is faithful. After discovering that Woodcock has a history of infidelity with a former spouse, he is convinced that this pattern will repeat with his mother. The rivalry culminates at the Cornival, where Farley finally defeats Woodcock in a corn-eating contest to the delight of onlookers, including his new love interest Tracy, who is simultaneously appalled by his childish behavior.
In a turn of events, Farley and Nedderman devise a plan to plant a camera in Woodcock's home to catch him cheating. However, their plan backfires when they inadvertently stumble upon an intimate moment between Woodcock and Beverly. During a ceremony where both Farley and Woodcock are set to receive awards, Farley confronts the crowd, denouncing Woodcock as the "biggest asshole on the planet." This confrontation leads to a dramatic fallout, ending with Beverly rejecting Woodcock after witnessing his true nature.
In a poignant conversation the following day, Farley comes to terms with his selfishness and realizes that he has always sabotaged his mother's happiness ever since the death of his father. After a reconciliation attempt with Woodcock, an explosive fight ensues, leading to an injury for Woodcock. As they visit him in the hospital, a fragile truce is formed between the three of them. Farley’s epiphany transforms his perspective on personal growth; he acknowledges that embracing one's past is the path to true self-acceptance.
Ultimately, Woodcock and Beverly tie the knot, while Farley rekindles his romance with Tracy. Inspired by his journey, he pens a new book entitled Backbone: The Definite Guide to Self Confidence, with a newfound understanding of life and relationships.
