The family curse has been lifted, and the Widdershins sisters—Betty, Fliss, and Charlie—are free to leave the misty prison isle of Crowstone. But when a mysterious girl arrives at their inn accompanied by a will-o’-the-wisp, or ghost, it seems another adventure has landed on their doorstep. The stranger, named Willow, is an escaped prisoner from nearby Lament, desperate to clear her father's name before he's executed for a crime he didn't commit. 

Then Charlie is kidnapped by poachers, swept away on a dangerous mission in search of pirate treasure that's protected by a vengeful ghost. Betty, Fliss, and Willow pile into their trusty boat and embark on another incredible adventure, one that involves ghosts, fierce pirates, and a journey to a mythic island that exists only in legends. Can they save their sister—and themselves—once more?