The Supermarket Guy III: Supermarket Dominium takes off from a peaceful time of robust growth for Harold's supermarket chain. With the evil Jon Acadou behind bars in a maximum-security prison, nothing seems to be threatening these prosperous and peaceful times. A new supermarket chain, the Edmundo Market, is just on the heels of Harold's own franchise. He soon finds out that it only takes a few days for his world to turn inside out.
Mr. Acadou escapes from prison, with his heart set on retaking the helm of the Edmundo Market after having his own one-sided, deluded religious conversion. He discovers much has changed with his foster son, Edmund, at the reins, and Edmund refuses to hand over control. Instead, Edmund plans on world domination with an engineered concoction that will make everyone duly follow him.
Suddenly, Jon Acadou needs the help of his old rival in order to stop him in his tracks as the police, military, and total population swing over to Edmund's thinking. All of Harold's friends now rush to abandon him. Harold and Acadou become wanted fugitives. Having lost everything, now Harold must trust and rely on an old foe who continues to accidently cause him near-death experiences along the way. A seemingly harmless bite from an experimental singing sheep gives Harold supersheep powers. But will it be enough to stop Edmund's bewildering plot to blow up the entire world under a total nuclear assault?