When 10-year old Coop's spoiled little sister Millie brings home a mysterious stray cat his idyllic life is turned upside-down. It turns out that this is no cat at all, but rather a malevolent space alien who just happens to look like a cat.
Coop falls for Old Lady Munson's niece Fiona, and Kat does everything he can to make Coop look like the juvenile delinquent Old Lady Munson claims he is.
Kat's swarm of flea-sized mind control nano-robots hop into the ears of the people of Bootsville, and soon they all think they'd like to tear Coop limb from limb.
Millie brings home her class hamster 'Mr. Cheeks' for the weekend, unaware that Mr. Kat hates hamsters, and Mr. Cheeks is a very special breed of rodent.
Coop tries to stop the annihilation of Massive Mart by a huge laser cannon sent to eradicate Captain Blasteroid, who the Kat Kommander thinks is a real live Earth hero.
While Coop and Dennis explore Kat's surprisingly spacious scratching post lair, Millie decides to toss it into a garbage truck, with the boys still inside.