Growing up in Hastings, Minnesota in 1970, young David Leary, Rick Moranis, was bullied by Roscoe Bigger, Fang, because of a pointed tooth. David is ecstatic when his parents announce they are moving to Oakland. David informs teachers about Fang stealing a Moon rock and Fang is arrested.
Twenty-six years later in 1996, David is divorced and raising his troubled son Ben as a single parent. A published author (albeit not a popular one), David jumps at an offer from his old school to teach creative writing for the fall semester. He meets wacko neighbors Art and Betty Lundstrum and begins rekindling a relationship with his old flame Victoria. He also encounters the school librarian Mrs. Rumpert who is still waiting for David to return Green Eggs and Ham to the library. After Ben begins picking on a kid named Kirby, David meets the boy's father Ross Bigger when both are called to the office of Principal Kokelar. Following a fire drill, David meets with his old friend Ulf, a fire fighter. When meeting with Ulf, Alan, and Gerry at a bar, David learns that after Ross got out of juvenile hall, Ross' parents skipped town which led to him growing up in an orphanage.
When Ross learns who David is, he resumes his old routine of bullying him to make himself feel better. David's son begins bullying Ross' son, but after a discussion, they become friends. Ross' intimidation tactics cause David to become paranoid, which unnerves another teacher named Clark, who thinks David is on crack. When David brings Ross' actions to Principal Kokelar after a recent pranking, the principal sides with Ross due to his seniority at the school and reveals that he has gotten some complaints from Clark about David. He warns David that if he can't straighten up his act, he will get another teacher to cover for him for the remainder of the semester.
Later that night, David and Ross meet at the old see-saw, where Ross reaffirms that David has never stood up for himself, while David admits snitching to get Ross put in Juvenile hall. After a game of cat-and-mouse in the school after hours, David flees to his old childhood hiding place in a cave. Ross chases him onto a waterfall and tells David that he always thought of him as a friend, before attacking him. David hits Ross with a piece of driftwood causing him to fall into the river. Fearing that he has killed his enemy, David tries to turn himself over to the police, only to find that the cops are out. Ulf drives David to his home while he tries to find Ross' body. After a talk with Art, David attempts to go to sleep only to ambushed by Ross. The two men fight once again until Kirby and Ben come in and reveal that they've made up and encourage their fathers to do the same. Ross reveals he stole the moon rock because he wanted to be an astronaut. It is also shown that during their recent fight, Ross' fang was chipped. They finally patch things up.
After finishing the semester, David begins to pack up to move to New York. He has Victoria return Green Eggs and Ham to the school library for him. Ross arrives and has a goodbye present for David: an Evel Knievel action figure identical to the one David had as a child before Ross threw it into a river. David invites them to visit, and the changed family leaves. Ross hooks up his mobile home to his truck, and follows David, telling his family they have been "invited" to come to New York and that's why they call it a "mobile home."
