Twleve-year-old private investigator Fletcher Moon, nicknamed "Half Moon" because of his shortness, must track down a conspiracy or be framed for a crime he did not committ.

There's a storm brewing over Eerie-on-Sea, and the fisherfolk say a monster is the cause. Someone has woken the ancient Gargantis, who sleeps in the watery caves beneath this spooky seaside town where legends have a habit of coming to life. It seems the Gargantis is looking for something: a treasure stolen from her underwater lair. And it just might be in the Lost-and-Foundery at the Grand Nautilus Hotel, in the care of one Herbert Lemon, Lost-and-Founder....