Miss Gordon and the Brownie Scouts enjoy walking the trails at the nearby park. One day while they eat their lunch, Veve and Sunny wish that a tree house would be built by brownie fairies in an old oak tree. They ask for a door, windows, and even a staircase. Nobody expects for a tree house to be built for the girls, so everybody is surprised two days later when they return to find a tree house in the old oak tree. Miss Gordon is uneasy, since the builder is unknown and the tree house is on public property. However, after the park superintendent gives permission for the girls to use the tree house, Miss Gordon finally agrees that the girls may use the tree house. The girls put up curtains and display dishes in the tree house. They make plans to come to the tree house on a regular basis. During the Brownie Scouts' trips to the park, the Brownie Scouts uncover several mysteries. Why does poverty-stricken Mrs. Myles refuse to accept any charity from nice Mr. Vincent? Who are the mean boys who vandalize the park? And who built the tree house for the Brownies? The Brownie Scouts have many exciting adventures as they learn the answers to these questions.