Foster's Home for Imaginary Friends - Season 6 Episode 8
Destination: Imagination (3)

Frankie is tired of working without being thanked. One day, a toy box is discovered at Fosters front door. Frankie is ordered to take it to the attic and leave it alone, but she knows there is an imaginary friend trapped inside, and opens the box to find it. When she looks inside, she falls into a strange imaginary world. There, she gets treated like a princess by a mysterious imaginary friend. After tackling certain bizarre toy box trials and tribulations, Bloo, Mac, Wilt, Eduardo, and Coco discover Frankies whereabouts and set out to rescue her from this strange world. This is the first and only episode to have a TV-PG rating.