A Murder of Convenience

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....