The Grim Adventures of Billy & Mandy

The Problem with Billy (4x9)

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When Billy goes into the mud for the fifth time in the day while wearing his white tuxedo before a wedding, Grim tries to figure out why Billy is so stupid, in other words, what is his "problem". Then, Billy's dad, Harold, recounts Billy's life from his birth until he met Mandy, and explains that Billy is not dumb, but actually a genius with a certified "Honorary Genius Degree". In the end, it is quite evident to Grim that Billy's problem is his equally dumb father.