Muffy is throwing a party at a water park, and everyone is invited, but Brain refuses to go. Arthur and Binky discover that the Brain is afraid of water and try to help him get over this fear so he can go to Muffy's party. In a letter to Buster, Arthur tells him that he is struggling with being funny when his friends act as clowns for a carnival. Arthur cannot learn any clown tricks, and his friends fear that he will ruin the carnival. Arthur learns that he has his own style of being funny that works: playing the piano while making faces with Kate.