The Welsh town of Tenby is disturbed by the local Militia discovering a young boy and a beautiful American girl, who have been deposited in the graveyard by a French balloon. The year is 1795 and Britain is at war with the Republic of France. The Boy (Richard O’Sullivan) is Louis XVII, son of the executed King of France. His protector, Virginia Traill, (Belinda Lee) seeks refuge for them both at her aunt’s home- a castle on nearby Caldy Island. Meanwhile, the Duc De Beauvais returns to France to rescue his own son Richard, who had been used as a substitute for Louis. Finding his son murdered, the Duc De Beauvais finds himself pursuing St. Gerard back to Caldy in order to save Louis and Virginia’s lives.