The Curse of Reptar All Grown Up! : Season 3 Episode 4

After Stu sells an invention, he tells the kids that he's splurging on a built-in swimming pool. While digging up the yard, the construction crew unearths the Rats' old mechanical Reptar toy. The kids fondly remember it, but decide to throw it away, until, during an electrical storm, they become convinced that they've angered Reptar. They ultimately decide to rescue the toy, and realize that, although they're growing up, they can't forget their past.