Monday with a Mad Genius Jack and Annie know they have to search for one of the four secrets of happiness to help Merlin the magician find joy in his life again. And so they head off in the magic tree house for Florence, Italy, in the early 1500s to spend the day with one of the greatest artists and inventors of all time–Leonardo da Vinci! Dark Day in the Deep Sea Jack and Annie know they have to find one of the four secrets of happiness to help save Merlin. But when the magic tree house whisks them off to a misty island in the middle of nowhere, they wonder how they will find anything. As the fog lifts, they see a huge ship that turns out to be the world's first floating science laboratory. The ship's crew is searching for undiscovered sea creatures–and for the giant sea monster that seems to be following them!