This is the story of an amateur detective, Lord Peter Wimsey, who sets out to prove the innocence of his elder brother, who has been charged with the murder of a Captain Cathcart, who happens to be the fiancé of their sister, Lady Mary.
The novel is a story set amongst the English upper-classes at a time when, if Dorothy Sayers is to believed, cheating at cards was thought to be worse than murder (and murder and adultery were equally serious transgressions) and where a Lord would rather face the gallows than sully the honour of a woman or bring scandal upon the family name. I suspect that the portrayal of that society was a caricature, even in 1926, when the book was originally published.
All of this lends a slight unreality to the proceedings, and Peter Wimsey is not a detective of the same calibre as, say, Sherlock Homes; for example his discovery of a vital clue (a letter stuffed into a rattling window) is serendipitous. Some of the characters are straight out of central casting, Wimsey's man Bunter is the equivalent of Jeeves, and the Dowager Duchess is a marginally more forbearing version of Lady Bracknell from the Importance of Being Earnest.
All of that said, this is an enjoyable read. The story moves at a good pace, there is action and comedy (the Dowager Duchess is particularly good value), and Sayers' use of dialogue to bring out character and move along the plot is skilled. There are some serious moments also (after all a death has occurred) and World War I, which was not long over, casts a show over the characters. If you are looking for a classic whodunit along the lines of, say, Agatha Christie, I would give this a miss. However, if you enjoy well written, tongue in cheek detective stories that give an insight into a very different social milieu (albeit that insight may be through a distorting lens that says more about how that society was perceived than about how it actually was) this is well worth a visit.