Tree House Mystery

The Alden children are staying at the New England farmhouse where their grandfather grew up. There they uncover a mystery about who really owns the nearby woods and the old tree house in it. A family moves into the house next door and the Boxcar Children decide to make friends with their new neighbors by building a tree house with them. In the process, they notice a window in their neighbor's house that nobody knows about, which leads them to suspect that the old home might have a secret.