The domineering Edwina Black has just died, and the general feeling appears to be of relief rather than grief. The local community whispers that her death is a disguised blessing for all concerned, particularly her henpecked widower Gregory and downtrodden personal companion Elizabeth. Unknown to anybody, Gregory and Elizabeth have been clandestine lovers for some time, and matters take a serious turn when the local doctor, feeling uneasy about Edwina’s sudden and unexpected death, orders an autopsy. The result come back, revealing that Edwina’s body is full of arsenic.

Inspector Martin determines to get to the bottom of the case and his suspicions obviously fall on Gregory and Elizabeth. In the absence of concrete proof of their guilt, he sets out to trap them, hoping that they will inadvertently implicate themselves. A complicating factor arises when it is discovered that the housekeeper Ellen has been keeping secrets of her own, and also had good reason for wishing Edwina ill. Two travel tickets and a guidebook to Italy are found in Elizabeth’s possession. How does she explain that away?

Martin proceeds to drop seemingly innocuous but loaded observations into the ears of the three suspects, hoping to provoke doubts and foster mutual suspicion. This works so well that they are soon apparently falling over themselves to incriminate each other. Martin has to try to untangle the stories to come up with a coherent picture of what actually happened, all the while being aware that he is perhaps being manipulated into barking up entirely the wrong tree.