Morgan wanted Jack and Annie to be Master Librarians. They must past the test to show they could research and solve 4 riddles.. She sent them to an island made of coral reefs. Jack was so worried to solve the riddle, but Morgan said mysteriously they would know it once they were there. The first riddle was, ¡®Rough and gray as a rock. Plain as plain can be. But hidden deep inside, there¡¯s a great beauty in me. What am I?¡®

When they got to the island, they found the land was pink because of coral reefs. Corals are tiny sea animals. After they died, their skeletons remain. Over time, the reefs build up from the stack of coral skeletons. On the shore, they found a mini-submarine which took them into the ocean. Jack and Annie saw the strange world which looked like a another planet. Coral reefs, starfish, jelly fish, sea horses , many kinds of fish, even a giant clam which they had never seen before. Suddenly two dolphins were looking at Jack and Annie outside the mini-sub.

Jack and Annie found ship¡¯s log, a journal of an ocean trip, and they realized the mini-sub was defective, which meant there were cracks in hull. So they tried to get back to the shore, but they met a octopus which hugged the mini-sub creating more cracks in the ceiling. Then a shark appeared and the octopus squirted black ink and ran away. Jack and Annie got to get out of the sub because the waster was spurting up to their knees and slipped the mini-sub and swam as calmly as they could do  because of the shark. While they were struggling back to the shore, Jack was so tired to keep swimming, and began to sink. But two dolphins pulled Jack and Annie toward the reef.

On the reef, Annie stepped on an oyster. It was rough and gray as rock and plain as plain can be. They just found the answer of the riddle. It was ¡®oyster¡¯. When they got back to the tree house, on the scroll, the riddle faded away, instead one shimmering silver word, ¡®OYSTER¡¯. It was Morgan's magic. On their way back home, Jack and Annie thought two dolphins were the true pearl in the oyster.