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.

With the help of the daring Violet Parma, ever-reliable Herbie will do his best to figure out what the Gargantis wants and who stole her treasure in the first place. In a town full of suspicious, secretive characters, it could be anyone!