Grab your hammer and nails, because the Alden family is building a brand new adventure. When a new family moves into the house right next door, Henry, Jessie, Violet, and Benny decide to make friends by helping them construct a massive, amazing tree house in their yard. But once the Boxcar Children climb up to their high perch, they see something completely unexpected. From the top of the tree house, they can look right into their neighbor's attic, and they spot a strange, round window that nobody knew existed! The old house is hiding a secret room, and the clues inside are linked to a missing spyglass and two brothers who have been feuding for years. Can you help the brave siblings uncover the truth? You will need your best problem solving skills to explore the hidden room and solve this high flying mystery.