Rose Beanie is going to be a sixth grader. The story begins during the last week of summer before sixth grade. Together, she and her three friends, Molly, Albert and Archer create a project that they call the best autumn adventure ever! During the autumn season of school, the four friends learn how to build a fort in Molly's backyard. During the building of the project, there is a surprise mystery that takes place. It is up to them to solve the mystery together. At the end of the autumn season, they share their project with the town and it is a real hit!