Joe Dirt (David Spade) is an irrationally optimistic, 30-ish, southern-rock and muscle-car-loving, mullet-sporting, lovable loser working as a janitor at an L.A. radio station. Finding Joe an irresistable target for his sarcastic on-air wit, a Howard Stern-like deejay named Zander Kelly (Dennis Miller), puts Joe on the air to tell his story. For three days Zander and a growing legion of listeners are surprised to find themselves captivated, delighted, and occasionally repelled by Joe's incredible tale of his search to find the parents who referred to him as "Joe Dirt" (rather than by his real last name, which he can no longer remember) and left him sitting in a garbage can at the Grand Canyon when he was 8 years old. Thorughout the film, the story reverts for short periods back to the KXLA studio for dialogue between Joe and Zander.
Pivotal to Joe's story is the past relationship he reveals between himself and Brandy (Brittany Daniel), a beautiful farm girl from his adopted hometown of Silvertown, Idaho. Joe feels that Brandy and his dog, Charlie, were the only real friends he ever had. After living in and leaving a series of bizarre foster homes, Joe met Brandy one winter when he was living in the woods outside Silvertown and Charlie's testicles became frozen to the front porch of Brandy's house. Brandy helped Joe and Charlie and for the next few years the three spent most of their time together.
Believing that Brandy is "too hot" to ever love a guy like him, Joe is blind to the fact that she obviously has very real feelings for him. Complicating their relationship is Brandy's alcoholic father (Joe Don Baker) and the local Trans-Am-driving bully, Robby (Kid Rock). Brandy's father kills Joe's dog after it fails in a Lassie-like mission to free the father when he gets stuck on railroad tracks, resulting in the loss of his leg.
The death of Charlie causes Joe to feel that he needs to take up the search for his mother and father. Joe is certain they did not leave him intentionally and must be looking for him too. His travels take him first to a town where Joe gets a job wearing a sandwich board advertising a local dental clinic. As he is working, Joe meets a kind older woman (Angela Paton) who is just about to put a mint condition 1969 Plymouth Hemi GTX convertible for sale. Admiring her car Joe tells her part of his story. Having recently lost her abusive police sketch artist husband to a tragic "accident" ("I pushed him into the woodchipper," she whispers), she sells Joe the car for the change in his pockets. She also gives him the idea of hiring a sketch artist to make pictures of his parents to aid in his search.
Before Joe can enjoy his dream car, he is sent to the roof of the dental clinic to secure a promotional hot air balloon. While Joe is in the balloon's basket, high winds cause the balloon to break free and Joe sails away. He is eventually knocked from the basket when it collides with an oil derrick in the Dakotas. Needing $360 to hire a sketch artist, Joe sees it as a cosmic sign that his search will succeed when he discovers the oil company is paying $360 dollars a week for work on the rig. Joe is a disaster as a rough neck and an outcast from the other workers, resulting in him being both fired after one week and beaten up by fellow worker Zeke (Lee Walker). Joe next spends the night in what he mistakenly believes is a circus tent. It is really a fumigation tent. The the next day, high on insecticide fumes, Joe wastes his $360 on sketches of what appear to be Father Time and serial killer Richard Ramirez.
Joe next decides to locate his parents through the use of an "Indian tracker." On a local reservation he encounters and befriends Kicking Wing (Adam Beach), the young proprietor of an unsuccessful roadside fireworks stand and aspiring veterinarian. Joe, who it appears is quite an afficianado on the topic of fireworks, encourages Kicking Wing to diversify into more spectacular wares than the "snakes and sparklers" he currently offers. While at Kicking Wings, Joe discovers what he believes to be an atomic bomb and, strapping it to his back with duct tape, uses it to blackmail a Grand Canyon tour bus company out of its list of passengers on the day his parents left him at the canyon. Joe intends to use the list to find people with photos that might show him or his parents. Joe gets the lists but the "bomb" turns out to be an RV septic tank and Joe ends up covered in rotten sewage.
Joe then goes to work for a traveling carnival as a means to travel the country tracking down people on the list. Back in the town from which he left in the balloon, he meets the seductive Jill (Jaime Pressley), a single mother who shares his affinity for southern rock, Hemis and mullets. Joe goes to the local impound yard to retrieve his GTX where a greasy attendant (Kevin Nealon) informs him the impound fee is over $3000. Unable to pay, Joe buys a rusty, fume-spewing 69 Dodge Daytona and informs Jill that his real car is unavailable because he's having a "footprint gas pedal installed." After making out with Jill, Joe discovers that she may actually be his long lost sister and he is faced with the decision of whether to go ahead and sleep with her. He discovers she is not in fact his sister and the rocking headboard indicates the two egage in some vigorous action.
At this point the story reverts from the past to the present and Joe makes a phone call from the LA radio station to Brandy. He leaves a message on her machine saying, "Hey, Brandy, it's Joe. I'm out in L.A. They got me on the radio station telling my story to everybody. I didn't get to the sad part yet. I'm guessing you had reasons for doing what you did. And I guess I'm just calling to say goodbye. I miss you, Brandy."
Joe then continues to tell his story to Zander. Joe leaves Jill to continue on his cross-country effort to track down every person on the bus list and look at their photos. This results in Joe being held prisoner in Indiana by "Buffalo Bob" a fishnet stocking weaing psychopath who intends to turn Joe's skin into an overcoat (ala "Silence of the Lambs"). Joe is rescued, and among Bob's photographs Joe discovers one which shows his parents' car with a Louisiana license plate.
Joe goes to New Orleans and takes a job as a janitor at an elementary school. Here he meets fellow janitor Clem (Christopher Walken), whose past is a little unclear. When a student accidentally makes "killer mustard gas" and the class is nearly overcome, Clems rescues them. In a clear attempt to avoid the spotlight he tells everyone it was Joe. At the resulting news conference Joe tells reporters of his search for his parents and also reveals that it was actually Clem who was the hero. That night mafia hitmen arrive at Clem's house. Joe comes to the scene the next morning to find police removing the dead bodies of Clem and his attackers. As Joe mourns over the covered body of Clem, Clem reveals to Joe that he is not actually dead but is being relocated again as part of the Federal witness protection program.
Joe takes a new job at an alligator farm and befriends the farm's owner (Rosanne Arquette) who inherited it from her parents when they were killed by a gator. She hates her life there and wishes a man would come along and take her away. Joe enthralls tourists with his death-defying act of placing his head into the mouth of a huge gator. This of course results in Joe nearly being eaten. As Joe is reviving from his encounter with the gator, he remembers that his real last name is "Nunamaker."
With this final bit of information Joe learns his parents were from Baton Rouge. Joe arrives at his parents' home to find it condemned and abandoned. A backwoods Cajun (Blake Clark) tries to remind Joe that "home is where you make it," but Joe is unable to understand him through the thick accent. Joe is heart-broken. As he is lying on the ground staring at the moon he realizes he had a home in Silvertown with Brandy and that he needs to return. He calls her and gets the answering machine. He leaves a message saying he is returning, but the message is deleted by Robby.
As Joe walks into Silvertown he is met by Robby and his toadies. Robby tells Joe that he is marrying Brandy and that Brandy found Joe's parents in California. He says Brandy doesn't want to see Joe. Joe doesn't beleive this but Robby shows Joe a note, signed by Brandy with X's and O's, saying she found Joe's parents but that Robby shouldn't tell Joe if he calls. Joe walks away pretending not to be crying while Robby continues to insult him.
The scene returns to Zander Kelly's studio. Zander is incensed that Brandy would treat Joe that way and insists on getting Brandy on the phone. Brandy is put on the air and Zander confronts her about her note to Robby. Brandy admits she wrote it but only because she wanted to tell Joe in person, and months went by and Joe never came home. Brandy then tells Joe that his parents were killed the day they were at the Grand Canyon. She says Joe should come home to Silvertown and let her take care of him.
Joe is next shown as having become a media sensation. Before returning to Silvertown he makes an appearance on the television show "TRL" with Carson Daly. While Joe is on TRL a women calls in and claims to be his mother. Joe believes the call is legitimate and he goes to a trailer in Simi Valley, CA, to meet his parents. Joe discovers that his mother (Caroline Aaron) and father (Fred Ward) are alive, but that they left him on purpose and have only contacted him to get publicity for their online business. Joe storms out of his parents' trailer.
Joe threatens to kill himself by jumping off a bridge in LA. The crowd and police implore him to reconsider and to come down from the bridge railing. Joe says that he has no one and that everyone lies to him. Brandy arrives and Joe confronts her about lying to him about his parents' death. Brandy admits the lie but explains that after she met his parents and discovered what horrible people they were she wanted to protect Joe from them. Brandy expresses her love for Joe and her desire to marry him and start a family.
At that moment Joe is roped by a policemen with a bungee cord. This causes Joe to fall off the bridge. The bungee cord breaks his fall, but as he rebounds he hits his head on the bridge and is knocked unconscious.
Joe awakens in Silvertown to find Kicking Wing, Clem and the alligator farm owner all standing by his bed with Brandy. Kicking Wing has become a successful fireworks entrepeneur based on Joe's advice and Clem has taken a new name and been relocated to Silvertown. He and the farm owner are planning to be married. Brandy has also given Joe a surprising new haircut and bought a new dog, "Charlie II."
Joe realizes he now has a loving family and real friends. The film ends with all five racing off in Joe's convertible, leaving Robby in a cloud of dust and gravel.