Mysterious footprints in the sand. A fallen map. Who has been visiting a secret sea-cave and what has been their business? Hobnail and her friends, Warty Toad and Snidey Slug, have a problem. They do not want uninvited guests - especially when they are sometimes strangely invisible. But Marnie McPhee and her loyal pi-rat, Romany Ratticus, have made a promise. A promise to drowned shipmates. More trouble comes Hobnail's way when Liar-nel, the Leaf Man, discovers pirate pretties are being stowed away in the Dark Hole. She will have to act quickly if she is to outsmart him and help Marnie to keep her promise. Because a pirate's promise is never broken.