Case

After arriving at his London club at 10:30 am precisely, which he has been doing every morning for many years, Sir Eustace Pennefather, a known womanizer whose divorce from his current wife is pending, receives a complimentary box of chocolates through the post. Disapproving of such modern marketing techniques, Sir Eustace is about to throw away the chocolates in disgust but changes his mind when he learns that Graham Bendix, another member of the club whom he hardly knows, has lost a bet with his wife Joan and now owes her a box of chocolates. Bendix takes the box home and, after lunch, tries out the new confectionery together with his wife. A few hours later Joan Bendix is dead, whereas her husband, who has eaten far fewer chocolates, is taken seriously ill and hospitalized (but later recovers). The police can establish a few facts beyond any doubt: that the parcel was posted the previous evening near The Strand; that the poison that was injected into each of the chocolates is nitrobenzene; and that the accompanying letter was typewritten on a piece of stationery from the manufacturers of the chocolates but not composed or sent by them.

Quite soon in the police investigations it becomes evident that the intended victim was Sir Eustace himself rather than the innocent Joan Bendix: no criminal could have predicted Sir Eustace giving away the box of chocolates to a man he hardly knew who just happened to be present when it was delivered. However, at a loss as to the further details of the crime, Scotland Yard conclude that the sender must have been some maniac or a fanatic trying to rid society of one of its most immoral members.

Theories

Worthy pillars of society including a barrister, a writer of detective novels, and a female author, the members of Roger Sheringham's Crimes Circle go about individually solving the case. After a week has passed, they present their findings on consecutive nights to their colleagues. Not surprisingly, they come up with various suspects: Sir Eustace's estranged wife; the father of a young lady whom Sir Eustace intended to marry after his divorce got through; one of Sir Eustace's discarded mistresses; and some more. At the very end of the novel, however, there is no doubt as to the identity of the perpetrator.