The Grim Adventures of Billy & Mandy

Terror of the Black Knight (3x25)

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After being impressed by Irwin's realistic armor at the medieval fair, Billy wishes for a fancy armor too. Then, Grim gives him the "Cursed Suit of the Black Knight". However, when Billy needs to go to the bathroom, the curse is revealed to be that the armor can't be taken off until the person wearing it beats someone in a competition. Therefore, Billy desperately tries to defeat other people at various contests but fails to come out victorious. In the end, Billy defeats Mandy at a breakdance contest, in which Mandy decided not to participate, but he doesn't need to go to the bathroom anymore.