Foster's Home for Imaginary Friends (2004)

S1E5 The Trouble with Scribbles

Bloo makes the impulsive choice to unlock a forbidden door at the retirement home for imaginary friends, a door that is strictly off-limits. This reckless act leads to the unwelcome arrival of a troublesome presence that wreaks havoc throughout the house. As chaos ensues, Bloo and his friends must confront the consequences of his actions and find a way to restore peace before it's too late.