In the process of discussing with her husband the plot of her latest novel, mystery writer Lady Wimsey finds the solution to a real-life murder. The novel is an unfinished 1930s manuscript by British writer Sayers, completed by Walsh on the basis of the outline.

Dashing detective Lord Peter Wimsey is caught up in the murder trial of mystery writer Harriet Vane. Her fiance has died of poisoning exactly as described in one of Harriet's novels -- so naturally she is the prime suspect. As Peter looks on, he not only falls in love with the accused but eagerly helps with Harriet's defense when the first trial ends in a hung jury. Will she be convicted and executed for the crime, or can he save her life and win...

This mystery finds Lord Peter Wimsey revisiting his first case from thirty years earlier when the descendant of Lord Attenbury begs his help in proving the ownership of a priceless cache of emeralds.