Ellen Cuthbert's husband, Randolph, is now the Earl of Keswick's heir. Their marriage is a sham, and Randolph's mistress, Lydia, is present at the house party. When she is found murdered in a locked room, all the evidence seems to point to Ellen. And how could the murderer have escaped the locked room except by witchcraft? Sir Hugh accompanies his cousin, a magistrate, to the scene of the murder. They investigate, appalled to find their childhood friend Ellen appears to be the chief suspect. Hugh's lack of prospects years ago prevented their marriage. Now if he cannot find the real murderer, there may be only one final service he can perform for Ellen to spare her a slow death at the end of the hangman's rope.