Detective Inspector Andrew MacAndrew Elliot of Scotland Yard's CID has been assigned to look into a case in the village of Sodbury Cross, where someone has been poisoning chocolates in Mrs. Terry's tobacco-and-sweet shop, somehow substituting strychnine-laced confections for the chocolate creams kept on the shop's counter. Sadly, eight year-old Frankie Dell died after he'd "wolfed down the lot" he'd bought, while the children of another family and their "maidservant" who had also picked up a half pound of the tainted chocolates became very ill. The young niece of local businessman Marcus Chesney, Marjorie Wills (for reasons I won't go into here), is the main suspect. As Elliot begins to work with the local police on that case, word arrives that Marcus Chesney is dead. After arriving at the Chesney home, the detectives hear a fantastic story: as it happened, Chesney's murder not only occurred in front of a small group of people, but that
"every one of 'em saw the murderer and followed every move he made."
Even more surprising is that "they can't agree on anything that happened."
What comes next is unlike anything I've ever read before.
Chesney, who has as a hobby "the study of crime," had earlier invited his niece, her fiancé George Harding, his brother Joe and a friend of theirs, a professor Ingram, to a "performance," a sort of "psychological test" to start at midnight. His helper Wilbur Emmet, one of the men who worked at the Chesney home, was to have a role in this scene, and after it was over, Chesney would have a list of questions that the participants were to answer, based on what they'd seen. George was to film the entire thing as well. What happens next went according to plan, except for the fact that Chesney was murdered and Emmet was found severely wounded outside next to a bundle of clothing and other props used during Chesney's little game. The problem is that the potential suspects were all together at the time, never out of sight of one another. When another murder occurs, a rather mystified Elliot turns to Dr. Fell, who is staying in a hotel in nearby Bath, enlisting his help to solve this rather baffling crime.
I mentioned earlier that the original UK title, The Black Spectacles, turns out to be more appropriate than its American counterpart. In a letter from Marcus Chesney written earlier to Fell which he doesn't hand over right away to the police, Chesney had noted the following:
"All witnesses, metaphorically, wear black spectacles. They can neither see clearly, nor interpret what they see in the proper colours. They do not know what goes on on the stage, still less what goes on in the audience. Show them a black-and-white record of it afterwards, and they will believe you; but even then they will be unable to interpret what they see."
As Dr. Fell says before he goes off to observe George's film record of that strange night, "that, together with what we are going to see and hear to-night, should complete our case."