The Promise of the Witch is a tragic, yet comic tale, of a grandmother and her granddaughter, fated to honour a centuries old blood oath, that ties their clan to the masts of the Royal Navy. From an ancient line of witch-mothers, these women fulfil their promise to defend the shores of Albion from invaders and defend themselves from old adversaries that would emasculate them - this clan of sea-witches, the Britanni.