Arthur and his classmate Muffy discover, to their mutual dismay, that they share the same birthday—and have already scheduled their parties for the same Saturday. With too many preparations made to cancel, their classmates are torn about whose party to attend. Arthur conspires with Francine and sends secret invitations calling everyone to his house—including Muffy—with a promise of a present too big to fit in a box. Everyone shows up, both parties merge into one, and Muffy declares it should happen every year. Mrs. Read gently suggests that next year, the party is at Muffy's house.