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 ... a sea monster! Will Jack and Annie wind up in the monster's clutches?