Foodfight! takes place in the "Marketropolis" supermarket. After closing time, the supermarket transforms into a city in which all the citizens are "Ikes", personified well-known marketing icons.
In the Ikes' world, heroic cereal mascot Dex Dogtective (Charlie Sheen) is about to propose to his girlfriend Sunshine Goodness (Hilary Duff), a raisin mascot, but she goes missing just before he is able to do so.
Six months later, in the 'real world', a Brand X representative called "Mr. Clipboard" (Christopher Lloyd) arrives at Marketropolis and aggressively pushes Brand X's range of generic products. In the world of the Ikes, the arrival of Lady X (Eva Longoria), the seductive Brand X detergent Ike, causes a commotion at Dex's club, the Copabanana.
Brand X products begin to replace previous products, which is mirrored in the Ikes' world with the deaths of several Ikes. After Dex's friend Daredevil Dan (Wayne Brady), a chocolate squirrel, disappears, Dex begins to investigate. After rebuffing Lady X's attempts to bring him to Brand X's side, Dex is locked in a dryer with Dan to be melted, but the two manage to escape. Dan and Dex find out that Brand X contains an addictive and toxic secret ingredient.
Dex and Dan attempt to initiate a product recall with the store owner's computer. A Brand X Ike cuts power just as they send the message. Dex then rallies the citizens of Marketropolis to fight the armies of Brand X in a massive food fight. The citizens win the battle through cunning use of the supermarket's electricity and lightning rods to protect their own buildings.
Dex rescues Sunshine, who had been held hostage in the Brand X tower, and escapes with the help of Dan. Mr. Clipboard then enters the Ike's world, and it is discovered that he is a robot controlled by Lady X. Lady X reveals that she had previously been the hideous Ike of an unsuccessful brand of prunes, and had been stealing Sunshine's essence to create a new brand. Dex and Sunshine defeat her, reverting her to her original form. With Brand X defeated and a cure found that revives the killed Ikes, Dex and Sunshine finally marry.