The second Lord Peter novel. Even opens with an allusion to the first. And from the vacation with which it opened, Lord Peter learns of another crime that sends him scurrying home: his brother the Duke has been charged with murder.
Charles Parker was sent down to deal with this case -- having used some pull -- and between the two of them, he and Lord Peter discover a number of things about the crime scene that mystify the matter. Meanwhile, Lady Mary is behaving oddly herself, and has since the night of the murder -- the dead man, Cathcart, having been her fiance. Not to mention that they find blood on her skirt.
Unfolding this tangle involves a cat charm made of platinum and gems, a letter bag, the reason why Lady Mary consent to marry Cathcart, the reason why Cathcart asked her to marry him, a visit to a Socialist club, an injudicious walk in a boggy area, Lord Peter's explaining to his brother that he's not playing at being a detective because it's not a game, a missing letter or two, the Dowager Duchess's figuring out how Mary's so ill, Parker's attempting to buy ladies' underwear in France, a melodramatic confession from Lady Mary, and a jealous man with dogs.