Joe Dirt is the janitor at a Los Angeles radio station. A producer drags him into the studio to talk live on the air with famous disc jockey, shock jock Zander Kelly.
Joe tells his life story. As a baby he had a mullet wig installed because the top of his skull had never formed. At age 8, he was left behind by his parents and sister at the Grand Canyon. He does not know his real surname. After growing up in a series of foster homes, Joe arrived in Silvertown, a small town in the Pacific Northwest, where he met beautiful Brandy and her dog, Charlie, and became target for jealousy from Robby, the town bully.
After Brandy's alcoholic father shoots Charlie dead, Joe decides to try to find his parents. He strikes up a friendship with Kicking Wing, an unsuccessful Native American fireworks salesman. In Indiana, Joe has an encounter with a skin cannibal named Buffalo Bob. This brings him unwanted attention from the media, but helps his search. He travels to Louisiana and works as a high school janitor with "Clem Doore", a former NYC mobster in the Witness Protection Program, with whom he becomes good friends. Joe discovers the address of his old family home and travels to Baton Rouge.
Listening to Joe's life story, both Zander and the radio audience initially find him an object of scorn, but Joe's kindness, his optimistic outlook on life, and his good-natured self deprecation win them over.
Eventually, Joe lands the janitorial job at the Los Angeles radio station, where he recounts how, after discovering his old home vacant and his parents long gone, he gives up the search and returns to Silvertown to be with Brandy. However, Robby informs him that Brandy found Joe's parents, but instructed Robby not to tell Joe. Robby shows a note from Brandy to prove it. Hearing this, Zander calls Brandy on the phone on air, to find out why. Brandy says she wanted to tell Joe in person, but never had the opportunity. Brandy tells Joe his parents were killed the day they were at the Grand Canyon; she pleads with Joe to return to Silvertown. Upset by the news, Joe stays in Los Angeles.
Joe is unaware that he has become a media sensation but he quickly discovers his newfound fame. An appearance on TRL with Carson Daly results in a phone call from a woman claiming to be Joe's mother. Joe realizes that she is indeed his mother and finds his parents' house, where he and the media discover that his parents intentionally abandoned him at the Grand Canyon, and that they reconnected with him in order to take advantage of his newfound publicity to boost their sales of clown figurines. Angry and sad, Joe storms out, cutting ties with his parents.
Joe intends to commit suicide, but Brandy appears and says that she only told him his parents were dead to protect him from their greed. She invites Joe to come home with her, saying he "was home all along." Before Joe can come down from the ledge, he suffers a head injury in a freak accident when a policeman tries to prevent him from falling using a rope.
Joe wakes up in Brandy's house, surrounded by his friends: Brandy, Kicking Wing (who now owns 30 successful firework stands), Clem (now renamed Gert B. Frobe), and Charlene (another friend who is engaged to Doore). Brandy got Joe a new braided wig following his head operation. Brandy has retrieved Joe's Hemi, and she has a new dog that Charlie fathered.
Robby drives up and tells Joe that no one wants him in Silvertown, no matter how famous he is. Clem threatens Robby, and Charlene insults his car. They all realize that they are like family to Joe. They ride off, leaving a frustrated Robby in the dust, his windshield broken by the stones thrown up by Joe's car. As they drive away, Zander plays a song for Joe on the radio, and fireworks go off in the sky (with special thanks to Kicking Wing).
Who intends to commit suicide?
Joe