When ten-year-old Coop Burtonburger's spoiled little sister Millie brings home a mysterious stray cat, Coop's idyllic life is turned upside down. Coop soon discovers that Kat is more than just a scrawny, purple, hairless pet; he is also a super-smart, conniving mastermind, and Kat's also an alien with a fanatical hate for Coop himself. Kat instantly declares war on Coop—to the death; the two fight constantly as Coop tries to stop Kat and tell the world about his possible plans, only to find Kat destroying the evidence, and putting the blame on Coop every time.