Trick or Treehouse is the third half of the 29th episode in season 2 of Dexter's Laboratory, which originally aired on March 4, 1998. In this episode, Dee Dee, after being kicked out of Dexter's laboratory one too many times, creates a place of her own where everyone except Dexter is allowed in.
Dee Dee is building her treehouse and takes everything in the house, except for certain pieces of furniture. Meanwhile, Dexter is in his laboratory working on an invention until he stopped by hammer pounding coming from outside. He goes to find the source, and he is stopped by his mother yelling as her hand is caught in the biscotti jar. She smashes the jar, shattering it to pieces. Dexter continues to find the source of the pounding, and is stopped by his father playing a gong and tinkling bells. Dexter, thinking that he can rest, is once again annoyed by the pounding. He goes outside and runs to the only tree in his backyard. Dexter interrogates Dee Dee is she's up there. She responds, and Dexter is fooled by her mask. Dee Dee tells Dexter about her "secret place" and yells at Dexter to leave.
Dexter goes to look out his window with a pair of binoculars and watches as Dee Dee invites Mee Mee and Lee Lee to her secret place. Mom then goes into the tree with a tray of milk and pie. Dad enters the tree with his radio. Dexter is envious of Dad ditching him, thinking that the two were "best buddies." He then watches as a stray dog rides a swing up into the tree. Dexter is annoyed by Dee Dee's new place. At night, the guests inside Dee Dee's secret place all come out and laugh in happiness. Dexter goes to the secret place dressed as a cardinal. Inside the tree, he sees a treehouse and heads inside. Dexter heads toward a lever and pulls it. A bread box falls on him, and Dee Dee goes to destroy Dexter's laboratory, much to her brother's dismay.
Dee Dee refers to all of the times her younger brother kicked her out of his laboratory.
The name is a pun on the saying "Trick or Treat", a phrase commonly associated with Halloween, and "treehouse".
Dexter's beak on his bird disguise does not move as he makes chirping sounds.