Lord Peter Wimsey's elder brother the Duke of Denver is accused of murdering Dennis Cathcart. Cathcart was engaged to the Duke's sister Lady Mary, and was shot in the Duke's garden during the night. The Duke himself was discovered near the body, and it was his gun that was used in the murder. Wimsey and Bunter arrive from the continent and start to investigate the crime, hoping to save the Duke from the hangman's noose.
Harriet Vane, a crime writer, is charged with poisoning Philip Boyes, her former lover, with arsenic. The evidence against her seems overwhelming, but it is all circumstantial. The jury is divided and a new trial is ordered, to begin in four weeks' time. Wimsey visits Harriet, finds himself falling in love with her, and sets out to clear her name.
Harriet Vane attends a reunion at her Oxford College and is asked by the Warden to investigate a series of poison pen notes that several members of staff have received. She takes up residence in the college and receives one of these notes herself. In the cloistered environment of the school, there is any number of possible suspects. In the absence of a motive however, it is difficult to determine if the culprit is a member of staff, a student or even if it is a man or a woman. When Lord Peter Wimsey visits the college - as much to see Harriet as for anything else - he agrees to assist in the investigation. What he finds is that one member of the staff had reason to seek vengeance.