The Mystery of the Tree House is the first story arc in the Magic Tree House series. In it, Jack and Annie go on strange adventures in the tree house. They have no idea who the tree house belongs to or how it got in the tallest oak in the Frog Creek Wood. These adventures are just for fun, as the kids are not on any missions. Each one of these adventures focuses on a different time period of history, first the Prehistory, then Medieval history, then Ancient history, and finally modern history in the Age of Piracy.
Morgan le Fay, the owner of the tree house, did not expect Jack and Annie to be able to use it. So she secretly accompanied them in various disguises, first as a Pteranodon, then a knight, then a cat, and finally a Parrot before revealing herself.