When 10-year-old Coop Burtonburger’s spoiled 8-year-old sister Millie brings home a strange 7-year-old stray cat named Mr. Kat, his idyllic life shatters as he discovers that the cat is actually a conniving, evil alien mastermind called a Catnipian, also known as a Kat Nebulan, a species of aliens resembling Sphynx cats from the planet Kat Nebula with a fanatical hate for mankind and an unassumingly sinister goal. The two battle daily as Coop tries to warn others of Mr. Kat’s evil, only to find his evidence destroyed by him and deemed a fool.