The people of Bootsville were murdered and taken from their homes, replaced with animatronic puppets controled by Kat Nebulans. The Kat Nebulans are actually living there on Earth and was in disguise all along. But Coop never noticed it, which seemed to be fun for the little monsters that took away his family and friends behind his back. Everyday was the same, Coop's friends and families bully him, which to the child seemed off. Ever since Kat came along, everyone acted differently.
Well, in the beginning, Coop went to get the baseball; resulting in Kat and Coop first looking eye-to-eye at eachother. Coop runs off to dad, leaving Millie to go look for herself. What actually happened in there, will make your guts wrench... Millie's screams can be heard, Burt and Coop was shocked and frightened, but Millie comes out happy with Kat in her arms. But, what we didn't see, was that Millie saw Kat as she went to pick up the baseball, but Kat grabs her arm, and bites her arm off! She shrieks in pain and fear, and that's what Burt and Coop heard, her shrill screams of Kat attacking her, and killing her. He then ate her face off, and took her body away for the rest of the sapient cat race to devour the corpse. They sent a robot that looks exactly a lot like Millie out to keep the Burtonburger family from getting suspicious.
But, how can the animatronic sleep and have Kat try and kill the robot? Simple! - Kat was playing Coop like a fiddle, he wanted to fuck with his life and send him to the nut house. If anybody wanted to drive this poor kid insane, Kat would want a slice of that pie.
What about Burt? He's not a puppet, he acts like a normal, goofy, all-American dad. Yes, but during the show, he's X'ed off during the entire show from "Under Destruction", to "The Kat Went Back part 1 and 2". It was simular to how the Kat Nebulans control the people of Bootsville in the episode, "It's All In Your Head". But they done it from inside their bodies. And the humans are machanical marvels.
Coop will never figure out that his family and loved ones are now his foes, and Kat's people are controling their motions and talking through them from inside their bodies. Much like the films "Invaders from Mars" and "Invasion of The Body Snatchers", but worse. Coop might soon figure it out in adulthood, we'll never know... We'll never know... Coop will never know... he's alone in this world, and what was left of Bootsville, is a hollow shell, of itself.