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 them! The two are soon rescued by a shipful of explorers and scientists. But the crew isn't looking for the secrets of happiness. The crew is looking for...sea monster! Will Jack and Annie wind up in the monster's clutches?