Jack and Annie continue their quest for the secrets of happiness—secrets they need if they’re going to save Merlin. But when the Magic Tree House leads them to a tiny deserted island in the middle of the ocean, they’re not sure who needs help more—Merlin or themselves! The brother and sister team are soon rescued by a ship of explorers and scientists. But that isn’t the end of their troubles, because the crew isn’t looking for the secrets of happiness. The crew is looking for . . . a sea monster!

Mary Pope Osborne takes young readers back to the mid-nineteenth century, a time of exploration and adventure. This paperback edition contains all-new backmatter full of activities, and the first printing includes temporary tattoos.