Arthur gets fired by his new piano teacher, Dr. Fugue, for neglecting to practice, and then must find a way to convince Dr. Fugue to give him a second chance.
Unable to think of an idea for a science fair project, Arthur borrows a model his father made in third grade but feels bad about it even before he gets into trouble.
Arthur's Grandma Thora, a marbles champion, trains him for the first-ever Elwood City Marbles Tournament, but does not tell him that she will compete against him.
Arthur and his friends try to find a substitute to attend the Father's Day picnic with Buster, whose father is not around much since his parents' divorce.
Mom and Dad are ready to go out for the evening, but Arthur and D.W. are worried that their surprise babysitter might be a pirate, robot, or even Mr. Ratburn.
Arthur and his friends face a number of dilemmas trying to get ready for school in the morning, and Arthur is both hindered and helped by his little sister.