Scotland, 1396: Euphemia MacPhee lives alone in a witch's cave with her healing herbs and an ancient skull for company. Her reclusive life shatters the day she learns that her four-year-old son, fostered on the Isle of Barra, has mysteriously fallen ill. Euphemia sails to remote Kisimul Castle to nurse the boy and soon discovers he has been poisoned. Why would someone strike at an innocent young child? Can she trust anyone there to help her fight this evil?

Euphemia must utilize every remedy available to confront ancient malice and long-buried wounds, as she struggles to save her son and to clear her own name.