When 10-year-old Coop Burtonburger’s (turns 11) spoiled eight-year-old sister Millie (turns 9 and would be 10), 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 smart, conniving, alien mastermind with a fanatical hate for Coop himself. Kat instantly declares war on Coop. The two battle constantly as Coop tries to stop Kat and tell the world about his sinister plans, only to find Kat destroying the evidence, and putting the blame on Coop every time.