Although this is the sixth Wimsey mystery, it's the first in which Harriet Vane appears. The book begins with Harriet Vane on trial for having poisoned her ex-lover. Pretty much every one is convinced of her guilt, but not Lord Peter Wimsey. Wimsey decides to see that she gets off, not merely in a technical acquittal, but in the only way that will not leave a shadow over her for the rest of her life, i.e. finding the actual culprit.
This is a good story with lots of well-drawn characters. I've long been a fan of Dorothy Sayers. She's so much better than Agatha Christie because her characters come to life as real people. Christie merely provides manikins to effect her intricate plot lines.