6x13 - The Grim Adventures of Billy and Mandy

There is a new student in the school, named Dora (parodying Dora the Explorer), who insists on getting people to open her lunch box. Mandy befriends Dora once she gets into a fight with the snobby Mindy. While noting something strange with Dora, Grim does not recognize her as Pandora until it's too late and she tricks Mandy into opening Pandora's box, releasing all kinds of plagues upon humanity. Mandy restores everything to normal, and captures Pandora inside the box, saying that nobody tricks her, and that when the time is right it will be Mandy herself who will unleash humanity's doom and destruction.