Slimy tentacles slither out of the toilets and tangle with 12-year-old AJ Zantony and his younger cousin, Jasmyn, while they're visiting Zala Manor, their great aunt's creepy old mansion. AJ's buddy, Vlad, the pirate rat, stops by and pleads for help in saving his species from the creatures invading the sewers. AJ, Jaz, and their friends decide to risk everything to assist Vlad, plunging deeper into sea monster lore and facing the mysterious legends firsthand. Is it worth sailing into an otherworldly storm and confronting evil scoundrels, scarred souls, and the most nightmarish beast imaginable? The dangers mount, increasing the chances that AJ and his friends will fall victim to the Legend of Monster Island.