One morning, Mac wakes up to find that he's unintentionally created another imaginary friend named Cheese. Madame Foster allows the same rules as there are for Bloo, but Bloo does not approve of his new brother. After many attempts to get rid of him, Cheese disappears on his own. Bloo realizes that Foster's could be a very dangerous place for one so stupid as Cheese. Can he find him before something happens or Mac notices Cheese is missing?