The year is 1297, a time of strife and dissension in English-occupied Scotland. Kyle Shaw, deputy to the sheriff of Ayrshire, is on the scene when the body of an English nobleman is pulled from the River Ayr. The drowning appears to be accidental, until a sprig of mistletoe in the dead man's throat points to murder. Because such a plant is reputed to possess powers of an arcane nature, suspicion falls on a dreaded sect whose members in ancient times performed rituals that included human sacrifice.