Dexter's Laboratory - SE1-EP09a

Dexter's Laboratory was an animated American children's science fiction comedy series created by Genndy Tartakovsky for Cartoon Network.

The series revolves around Dexter, a genius kid with a secret lab filled with a collection of his inventions. Dexter is constantly in conflict with his sister Dee Dee, who always manages to enter his laboratory despite his efforts to keep her out of it, as well as Mandark his archrival.

Summary (with spoilers)
Dee Dee "borrowed" (without permission) one of Dexter's toys (specifically a robot with the explicitly labeled Property of Dexter). So Dee Dee uses that toy to make it participate in a love relationship game with a "Barbie" doll.

Dexter finds out about this and decides to retrieve his toy stealthily. For this, it occurs to him to miniaturizarse to pass unnoticed. However, the computer warns him that after miniaturization he will have hallucinations:

Dexter walks under the door of Dee Dee's bedroom (one of the advantages of miniaturization), approaching a pile of toy cubes that begin to see those as buildings. However, Dexter is still aware that he is hallucinating, so proceed with climbing that pile of cubes.

Unexpectedly, the cube at the top was a surprise box, which sent him crashing down in front of his sister Dee Dee.

Dexter catches his attention by picking up his toy, but Dee Dee shows him that his toy makes a cute couple with her "Barbie" doll.

Dexter sees his toy as a rough blue-gray version of himself again, but Dexter still realizes that it is a hallucination.

Then Dexter is forced to play in the dollhouse. He looks at the plate that is served on the toy table and notes that the snack is a toy, but after seeing more carefully begins to think it is real. During this hallucination, Dee Dee places the doll in front of him and Dexter, who still continues to hallucinate, believes that the doll is his partner.

Then, Dee Dee forces Dexter to be in a car for a few moments, then return to the house and realize that the doll "Barbie" was cheating with "Ken" (male doll of the same series of toys). Dexter fights against him and defeats him by ripping out an arm.

Dexter, angered by the betrayal, takes a ride in his car until he reaches a "cliff" overlooking the city (which is actually his sister's room). Then Dee Dee makes a joke that a ghost is talking to him (who is actually a clown puppet), so he decides to go home.

When he returns, he sees that he has been betrayed again, this time with the toy robot he wanted to recover (seen as a blue-gray and rude version of himself), so he decides to fight with him until he destroys it.

Suddenly, Dexter grows to his original size and notes that he hurt his "beloved" (beheaded, but dexter will have given another interpretation), that is, he continues to have hallucinations.

Dee Dee gets angry with Dexter for destroying the toy house and she is going to complain to her mother.

When they arrive at Dexter's room to reprimand him, they see that he treats the doll as a human being and they decide to retire after seeing that this child is grilled.

Trivia

 * Although in this episode it was shown to Dexter having hallucinations and that it fought them with its reasoning; at Dexter's Laboratory - SE1-EP06c, Dee Dee, being a giantess, also has hallucinations, but not because of her big size, but because of her own stupidity.