Dexter's Laboratory - Season 1 - Eps 38: Monstory
Not interested in hearing Dee Dee's long-winded story, Dexter gives her a silencing formula to keep her quiet, but it instead turns her into a giant monster still bent on telling her story. When he realizes that nothing can make her quiet, he becomes a giant monster himself.
About Dexter's Laboratory

Title: Dexter's Laboratory
First Air Date: 1996-04-28
Last Air Date: 2003-11-20
Status: Ended
Rating: 7.7/10 (from 813 votes)
Language: EN
Seasons: 5
Total Episodes: 220
Network: Cartoon Network
Genres: Animation, Comedy, Sci-Fi & Fantasy
Production Companies: Hanna-Barbera Productions, Cartoon Network Studios
Synopsis
Dexter, a boy-genius with a secret laboratory, constantly battles his sister Dee Dee, who always gains access despite his best efforts to keep her out, as well as his arch-rival and neighbor, Mandark.
Cast

Candi Milo
Dexter (voice)

Kat Cressida
Dee Dee (voice)

Jeff Bennett
Dad (voice)

Kath Soucie
Mom (voice)

Eddie Deezen
Mandark (voice)