In a cell of the Tower of London, where Louis (Dennis Price), the Duke of D'Ascoyne writes his memoirs. When Louis discovers that he is a distant heir to the title of the family but denied his birthright by the machinations of his late mother's in-laws, he plots to murder the other eight heirs (all played by Sir Alec Guinness) both as an act of revenge and his way to the dukedom. By luck or by his own hand, he assumes the title, but complications ensue when an old flame from his past reappears.