Monstory is the third part of the 13th episode in season one of Dexter's Laboratory. In this episode, Dexter gives Dee Dee a potion so he doesn't have to listen to her story. The potion, however, turns Dee Dee into a monster.
The episode begins in space featuring a giant multi-lens microscope reflecting an eye looking over the world and scaring the human population. Immediately in the next scene it is revealed that it is Dexter who is looking through his microscope at the world with an earpiece to listen to the sounds emitted by the planet in his laboratory.
While studying Dexter drops a pin whose sound after hitting the floor is amplified by his earpiece, hurting his eardrums. Dee Dee then shows up and despite Dexter telling her he needs quiet she tries to tell him a story. At one point Dexter asks Dee Dee if this is one of her knock knock jokes but she says no. When Dee Dee keeps talking Dexter gives her a potion he believes will shrink her mouth but realizes too late that it will turn her into a monster.
When Dee Dee begins crying, Dexter won't let her cries destroy the lab so he declares war and drinks the last drop of the potion. But Dexter just turns into a slug and Dee Dee chases him demanding he listen to her story. Dexter manages to lose Dee Dee and forms into a cocoon. Months later Dexter emerges as a monster too but Dee Dee, who has been sitting next to the cocoon patiently for months still wants for him to listen to her story.
Dee Dee chases Dexter into the city and Dexter and Dee Dee both mutate into even bigger monsters, the last one leaving them bigger than the city. Dee Dee changes into a giant spider on her own, while Dexter desperate to overpower her drinks toxic waste. They both eventually engage in a video game-esque fight that ends with Dee Dee pinning Dexter down to the ground. Dee Dee victoriously declares that now Dexter has to listen to her story and continues stating "so the boy told the girl in the park on the pony... Knock Knock!" Dexter realizing that the whole story really was one of Dee Dee's knock knock jokes lets out a loud "NOOOOOO!" with Dee Dee still pinning him. Just then the scene pans out showing the whole world and a giant multi-lens microscope from the beginning of the episode watching Dee Dee and Dexter.
The title of this episode is a pun combination between the words "Monster" and "Story".
Dee Dee mentions having a friend named Lila.
It is shown through a calendar that months had past during Dexter's metamorphosis, and Dee Dee was sitting right next to him waiting this whole time, Wouldn't their parents have noticed that children had been gone for a long time and get very suspicious.
The episode is a parody of the popular kaijun monster movies seen in Japan with both Dexter and Dee Dee parodying two monsters from the Godzilla franchise:
When Dexter and Dee Dee start attacking each other, for a second, when Dee Dee grabs Dexter's body with her arms, they turn purple, the same color as Dexter's skin. Then they turn back to the normal color.
This is the last episode of Season 1 of Dexter's Laboratory.
The character and background designs of the show are slightly changed after this episode.