From: The Oxford Book of English Detective Stories, ed. Patricia Craig. Oxford Univeristy Press, 1990.
Sir William Anstruther went to his London club for lunch, as he did every day, at half past ten. Waiting for him in the mail was a small box of chocolates, along with a letter stating the this was a new product designed to appeal to men, and asking for his opinion. Sir William was rather a man's man and was quite prepared to bin the lot when another member, Graham Beresford, happened by. In the end the chocolates went home with Beresford, to the delight of both men.
Once at home, Beresford had a few of the chocolates before leaving to attend to some business. Upon arriving back at his club several hours later he was taken gravely ill.
His wife had a few of the chocolates, and then a few more, and by evening she was dead.
All this was brought before Roger Sheringham, occasional consultant to Scotland Yard. He could make no more of it than the police. To the essential question, Who would want Sir William dead?, there seemed no good answer. Although the investigation went on, the general feeling was that this was the act of a lunatic, someone unlikely to ever be uncovered.
Until Roger had a chance meeting with a very silly woman on a busy London street. This woman, an acquiantance of his and of the unlucky Mrs. Beresford, mentioned a small fact in passing, the significance of which she did not recognize, though Roger saw it at once. And through tugging on that tiny scrap of string, he unraveled the entire mystery.
One of the greatest short stories of the Golden Age of detection (think Christie, Sayers, et al), "The Avenging Chance" has been reprinted many times, and appears in many anthologies of the best such stories. It strikes an excellent balance between dismay as such a callous crime, and a certain intellectual airiness in treating it largely as a puzzle. To a modern reader it's not as old-fashioned and windy as many of its ilk, and is certainly a landmark of mid-twentieth century crime fiction.