Their captain's insane vision of a Northwest Passage has kept the crewmen of HMS Terror trapped in Arctic ice for two years without a thaw. But the real threat to their survival isn't the ever-shifting landscape of white, the provisions that have turned to poison, or the ship slowly buckling in the grip of the frozen ocean. The real threat is whatever is out in the frigid darkness, stalking their ship, snatching and brutally killing their fellow seamen....

When Sepron the Sea Serpent promises to bring floods, starvation, and fear to the people of Avantia's coast, Tom and Elenna must figure out a way to stop the terrifying beast.

The gang abandons their plans for a relaxing vacation when a scaly sea serpent surfaces.

On a special after-hours tour of the Coolsville Aquarium, the gang tries to catch the Fishy Phantom, who has stolen the

"While playing a game of Pirate's Eye high in the crow's nest, Percy spots two giant tentacles in the ocean! He and Piper work with Captain Sharktooth to figure out what the strange creature is."