Sir Eustace is sent a sample box of chocolates to his club. He is disgusted anyone would send him something so trite, especially since he doesn't like chocolates. Next to him is Mr. Bendix. Mr. Bendix has lost a bet with his wife and owes her a box of chocolates. Might he nab the box and give it to her? Why not save a few pounds, he figures.
Sir Eustace gladly gives him the chocolates and Mr. Bendix takes them home and gives them to his wife who proceeds to help herself, insisting that her husband eat a couple, which he does out of politeness. He does not care for chocolates.
On his way back to work, Mr. Bendix becomes violently ill and ends up in the hospital. When he finally is released, he discovers his wife is dead.
The chocolates were laced with poison and the immediate conclusion is, who was trying to kill Sir Eustace? After all, they were sent to him.
The police investigate but come to a dead end.
Enter a criminologists club. This is a group of men and women who solve murders as a hobby. One member is a lawyer, but the rest are mystery writers.
What ensues is a couple of chapters devoted to each member as they solved the crime. Not only do they solve it to their satisfaction, but they also demolish each other's theories to their own satisfaction.
However, one of them turns out to be right and the ending is satisfyingly unexpected.