After a break from their trips in the tree house, Jack & Annie are delighted to find that it`s back! Morgan le Fay needs their help, but they`ll have to become Master Librarians first, & that means solving four riddles. For the first one, Morgan sends them deep into the middle of the ocean. Jack & Annie ride in a mini-sub & meet jellyfish, sea horses & an octopus. But there are some scary sharks in the deep & the friendly dolphins have to come to the rescue...