Alleyn is tasked by his Scotland Yard superiors with meeting French police colleagues to discuss international drug trafficking through Marseilles.

On the overnight sleeper train from Paris, the Alleyns witness what appears to be a fatal night-time stabbing in the illuminated window of a dramatically-set mediaeval castle overlooking the railway line.

This proves to be the resort of an élite, louche group of socialites who are dabbling in Black Magic under the auspices of a smoothly dubious host and 'high priest' of a cult that clearly involves drug-taking, with vulnerable wealthy women potentially exploited.

Garbel, a chemist she has never met who sends Troy unusual letters from the Maritime Alps which talk about, among other things, bus routes.

As the their train nears Roqueville, Alleyn and Troy spot a woman falling backwards in terror while a white-robed man stands over her with a blade in a window of a house.

An attendant confirms that the house is the Château de la Chèvre d'Argent, owned by a M. Oberon, the alleged leader of this drug ring.

Miss Truebody, another passenger, suffers a perforated appendix and the Alleyns agree to take her to the Chèvre d'Argent as the only nearby doctor, an Egyptian named Dr. Baradi, is staying there.

Examining the corpse, Alleyn finds a cavity under the left breast and immediately recognizes the body as the woman he saw murdered at the beginning of the novel, not Miss Truebody.

"[3] Philip Day wrote in The Sunday Times, "For her villains... Miss Marsh has fallen back on a very tired pair of stock characters; luck and guesswork take the place of detection, and the whole is a grave disappointment.

"[4] The Times delivered a mixed review: "Spinsters in Jeopardy is not quite a vintage murder, and Miss Ngaio Marsh, returning to the theme of esoteric religions, has not quite recaptured the élan and plausibility of Death in Ecstasy.

"[5] Maurice Richardson, reviewing in The Observer, wrote: "A read can always be guaranteed from Miss Marsh but you will need to hang your disbelief on a Upas tree before opening her this time.

"[7] In a critical essay on Marsh, Jessica Mann wrote, "Spinsters in Jeopardy... like the pre-war Death in Ecstasy uses the peculiarities of a lunatic and vicious religious sect as the peg for its plot... From this theme, as from others, Marsh retreats from full exploration; she is amused and sardonic, even a little disgusted, but not really interested, and the irrational urges of the devotees, the corrupt motives of those who batten on them, are seen wholly through a policeman's eye.

"[9] Margaret Lewis strikes a similar note: "His vocabulary is remarkably well developed for his age, and the precociousness... strains the reader's credulity.

[16][17][18][b] Lewis also says that Marsh based the sinister chateau on a Saracen fortress at Èze where she and her lifelong friends, the Rhodes family, holidayed in 1949.

In the words of Marsh scholar Bruce Harding, the title finally decided on reflects the fact that the novel features "three unmarried women in varying degrees of jeopardy".

[25] Earl F. Bargainnier notes how much the novel (with its murder occurring very early in the story, the kidnapping of Ricky, and the action climax) departs from "the classical formulas" of Golden Age crime fiction, calling it "Hitchcockian".