8-year-old Maggie Simpson steals her 16-year-old older sister-in-law Joslin Smith-Simpson's diary before she has to leave for an animated movie-themed summer camp, and Joslin now has to team up with her new group of friends to retrieve her diary. The story follows Maggie as she records her own thoughts in Joslin's diary from threatening to ruin her birthday party to scaring her family with a giant plastic spider to crashing her brother Bart's wedding, while Joslin prepares to start high school, gets backup from her friends to deal with Maggie and participates in activities like attending a movie-themed summer camp, going swimming, spending Halloween in California, getting her learner's permit, taking a trip to Meepville, volunteering to help her former preschool with Maggie's Christmas play and being her older sister Dot Warner's bridesmaid
