Shaggy and Scooby get a very unexpected new houseguest—Phibes! It seems Evil Phibes, his nastiest clone, has kicked him out of his lair and taken over. Although a broken Phibes promises to be on his best behavior, he proves to be a very inconsiderate roommate – he trims his toenails in the living room, he wears Shaggy's pajamas, and he makes prank phone calls to his doppelgänger that get him mad enough to obliterate the world with one of Phibes' own devices! Can Shaggy and Scooby rid themselves of this pest and keep evil Phibes from blowing up the planet in the bargain?