On the first day of summer, slacker high school student Dexter Reed takes his mother’s car on a joyride while she is away on a business trip but accidentally crashes into and damages both his mother’s car and the car of his teacher, Mr. Wheat. Dexter is in danger of going to jail, as he neither has a driver’s license nor car insurance. Fortunately, Wheat agrees to let Dexter pay for the damages in exchange for not calling the police on Dexter. With the damages estimated at $1,900, Dexter is forced to take a summer job. After being fired from the new and soon-to-open Mondo Burger restaurant for clashing with the owner and manager, Kurt Bozwell, he ends up working for Good Burger. There, he meets and reluctantly befriends the dimwitted yet charming cashier Ed alongside other colorful employees. Initially, neither of them are aware that Ed inadvertently caused Dexter’s car accident. Ed had skated in front of Dexter on his way to make a delivery, causing him to swerve and crash into Mr. Wheat’s car.
The survival of the smaller Good Burger is threatened by the grand opening of Mondo Burger, with its fancy decoration and oversized burgers. Luckily, Good Burger is saved by a new secret sauce created by Ed. Upon realizing that Ed caused his car accident and learning from Mr. Wheat that the damages from the accident exceed the original $1,900 estimate, Dexter takes advantage of Ed’s gullibility to extort money from him so that he can pay off his debt sooner. Ed promptly signs a contract that gives Dexter 80% of his profits. Dexter then tells Ed not to tell the secret recipe for the sauce to anyone.
Ed’s sauce vastly increases Good Burger’s sales but draws the attention of Kurt, who wants it for Mondo Burger. After failing to lure Ed to Mondo Burger at a higher wage, Kurt sends Roxanne, a beautiful employee, to seduce Ed into revealing the sauce recipe. As a result, Roxanne is repeatedly injured by Ed’s clumsiness and ultimately quits her job.
When a dog refuses to eat a discarded Mondo Burger in favor of a discarded Good Burger, Ed and Dexter become suspicious and decide to investigate. They infiltrate Mondo Burger’s kitchen and discover that their burgers are artificially enhanced with Triampathol, an illegal food chemical. Kurt discovers them and calls an acquaintance named Wade, who has them committed to an asylum known as Demented Hills so that they can’t bring to light their misdeeds.
Afterward, Kurt and his men break into Good Burger and finds Ed’s secret sauce then tainting it with shark poison. Otis, an elderly Good Burger employee who was sleeping on the premises, catches them red-handed causing Kurt to commit Otis to Demented Hills as well. After Otis informs Ed and Dexter about Kurt’s scheme, the three escape Demented Hills and hijack an ice cream truck to head back to Good Burger, arriving just in time to prevent anyone from eating the poisoned sauce.
Ed and Dexter then break into Mondo Burger to expose their chemically induced burgers to the police. While Dexter creates a distraction, Ed tries to take a can of Triampathol but clumsily knocks one can into the meat grinder. Inspired, Ed pours nearly the entire supply into the grinder. As Kurt corners Dexter, Ed suddenly arrives with an empty can. Kurt mocks Ed’s presumed foolishness, whereupon Ed snidely comments that the can wasn’t empty when he found it. Chaos then ensues in the Mondo Burger building, as the burgers begin to explode due to the overuse of Triampathol. A large artificial burger falls from the rooftop of Mondo Burger building which ironically destroys Mr. Wheat’s newly-repaired car.
In the aftermath, Mondo Burger is shut down and Kurt is arrested for using the illegal substance and contaminating Good Burger’s sauce. Dexter tears up the contract with Ed and tells him that he gets to keep all the profits from his sauce. Ed and Dexter head back to Good Burger, where they are both welcomed by the other employees as heroes for saving the restaurant.