"Wickedness! Such wickedness!" a dying woman gasps out as she makes her last confession to a priest. Before the priest can make it back to his home, he's coshed to death. The police find a list of names in his shoe (he has a hole in his coat pocket). After some research, the police realize that everyone on the list has recently died—of what is consistently judged to be natural causes!

Meanwhile, historian Mark Easterbrook and his date are having dinner with another couple. They are jocularly discussing the conveniences of being able to ring Harrod's and request a couple of murderers, when his friend's date, Poppy, mentions that "The Pale Horse" is where someone can go to get rid of a person. The following day when Mark asks her about it, she's scared stiff and refuses to talk about it. Could there be something to that?

And yet later, Mark attends a country fete in Bournemouth and discovers the mysterious Pale Horse is a local pub, run by three self-professed witches. They seem silly and dramatic ... but why was Poppy so scared?

Finally, Mark gets onto the trail of an organization that "bumps people off" for a price. The organization is centered around the witches that run the Pale Horse. Have they discovered some witchcraft or science that can cause people to develop diseases and die from apparently natural causes? Mark and a young woman named Ginger—with Mrs. Oliver helping a little too—are determined to get to the bottom of it.

Spoilers ahead! Scroll down for full spoilers of this book.

The force killing people is not witchcraft or rays from a scientific box, but plain old thallium poisoning. Because as far as anyone knows, the person is not being poisoned, and because the effects of thallium poisoning are not that well known, doctors assign some natural disease as being the cause of death.

The organization that bumps people off works something like this:

• A customer meets with a debarred lawyer who sets up the terms—how much you'll have to pay and how soon the person must die. The lawyer sends you to Bournemouth to the Pale Horse.
• The witches at the Pale Horse put on a show for the customer that seems to be a combination of superstition and science. As far as the customer knows, that's that.
• Actually, a third party visits the home of the person who's supposed to die, and substitutes a product containing thallium for a product the person usually uses—e.g., toothpaste, aspirins, etc.

How does this third party know what brand of product the victim uses? He has his own firm of canvassers who circulate about getting answers to questionnaires. The dying woman at the beginning of the book noticed that often a person she'd visited with her questionnaire, died soon afterward. She began to suspect something wicked was happening.

Mark puts the pieces together when he realizes that all the victims have one thing in common: they lose their hair, which is a symptom of thallium poisoning.

And who's the brains behind this operation? Little Zachariah Osborne, the proprietor of a neighborhood pharmacy. He's the third party who substitutes a poisoned product for a normal one. Osborne is nailed when he tries to set someone else up as the person behind the Pale Horse organization—the police catch him planting thallium packets in the person's gardening shed.

The book ends, satisfactorily, with a marriage proposal from Mark to Ginger.

This is one of the few later Christie books that deals with a type of gang or organized crime. The set-up of the organization is very clever and makes you wonder if there might really be such organizations in the world. The apparent central figures of the organization—the three witches—are interesting and bring out a theme of witchcraft versus science versus common sense.

The story is told as a combination of an omniscient third-person voice, and a first-person narrative by Mark Easterbrook. Mark is an entertaining narrator; his frustrations with getting someone to help him investigate the Pale Horse, and his romances with both Hermia and Ginger are fun to read. The combination of story-telling styles work well to show you the different clues that finally come together at the end to reveal the solution.

As a mystery, I think the clues in the story come from Christie, not from circumstances or the story itself. Christie's characters discourse extensively on traits of criminals (how they cannot leave well-enough alone and are essentially vain and need attention). The character that best fits these traits is Zachariah Osbourne, and this is ultimately what leads the police to have him tailed and finally catch him. The Pale Horse organization is well thought out, but I don't think the way it gets revealed and brought to justice is necessarily that ingenious.

The story, though, is really fun to read and interesting. Overall, I like this book a lot.