Susan Elia MacNeal’s eighth Maggie Hope mystery opens with a scene tailor-made for readers in the post-election, #MeToo era. During combat training, British secret operative Camilla Oddell kills a man with her bare hands and enjoys it. At the moment she snaps his neck, “a sinister, intoxicating joy flowed through her.” She bends over him with the triumph of a hunter inspecting his prey.

Her commander is not terribly surprised a wealthy society girl could kill so easily. “I’ve always suspected debutantes’ hidden capacity for violence,” he tells a colleague, adding later, “Thank heavens women in civilian life have no idea what they’re capable of.”

“The Prisoner in the Castle,” by Susan Elia MacNeal (Bantam)

It is the kind of gleeful celebration of women’s rage and power that is showing up in novels and on television right now — think Sarai Walker’s “Dietland” — but this novel is not set in the modern era. MacNeal’s series, which debuted in 2012 with “Mr. Churchill’s Secretary,” takes place in World War II and features the decidedly less bloodthirsty Maggie Hope, who manages, over the seven installments leading up to this one, to serve as secretary to England’s prime minister, unmask spies, thwart murderers and untangle her own complicated past.

Clearly, Maggie Hope can hold her own. But the introduction of seasoned killer Camilla Oddell offers the tantalizing possibility of an invincible dynamic duo. At the conclusion of the previous novel, “The Paris Spy,” Hope is locked in prison because she knows too much. Who better to help her break out and resume her life of crime-fighting and espionage than Oddell, sent to the same prison because of her violent proclivities?

MacNeal has other plans for “The Prisoner in the Castle.” The book is a locked-room mystery in the spirit of Agatha Christie’s “And Then There Were None.” There are several references to Christie’s 1939 classic, a book Christie wrote, according to her autobiography, “because it was so difficult to do that the idea had fascinated me. Ten people had to die without it becoming ridiculous or the murderer becoming obvious.” Her idea was that by the end of the novel, everyone would be dead, leaving a massacre scene for Scotland Yard to puzzle over. She required an epilogue to explain how it was done.

In MacNeal’s version, our protagonist, Maggie Hope — and now, Camilla Oddell and several others — are imprisoned on an island, much like the guests invited to an island holiday in Christie’s novel. It is helpful to surround them by a frigid body of water so that no one can escape, and it is also handy to have a reason they cannot send for help: In both novels, a storm makes it impossible for boats to approach.

There is one critical difference between Christie’s novel and MacNeal’s. “And Then There Were None” introduces readers to 10 characters who are alleged to be murderers themselves. Some of them might deny having committed the crime, or insist the death was not their fault, but even the possibility of their guilt absolves readers of the obligation to mourn for them. It makes it possible for 10 characters to die over the course of just under 300 pages without much sentimentality getting in the way.

“The Prisoner in the Castle,” however, has to get by without that device. Maggie Hope and her fellow inmates have, for the most part, served their country faithfully. Some knew too much, some were susceptible to bribery, and some had committed a moral but not a criminal offense. In fact, Camilla Oddell, the savage executioner from the opening scene, appears to be the most dangerous among them, although she is confoundingly well-behaved once it becomes clear a serial killer lives among them.

The author Susan Elia MacNeal (Noel MacNeal)

This is the difficulty with killing off a succession of characters who might not deserve to die: There is no time to mourn them as they go. MacNeal does her best to give Maggie Hope her moments of grief, but those feel superimposed upon a story structure that is built for speed, not emotional depth.

Of the plot itself I do not dare give away more. Another note of genius in Christie’s locked-room mystery is the characters, being trapped on an island, have nothing to do with their time but to sit around and discuss the case. This is useful, because it sidesteps the awkwardness that can occur in murder mysteries when the plot becomes so convoluted that a couple of characters have to sit down and explain it to one another, just to make sure the reader is keeping up. Here it is the entire point of the book.

This is how MacNeal’s characters put in most of their time, too: Between the murders and the discussions of the murders, there is not room for much else. At one point, Maggie Hope even makes a list of all the dead bodies, with names, dates and causes of death. She needs the list to make sense of it all, and I did, too. If you love a tricky puzzle that requires you to keep track of multiple alibis over time, this is your summer read.

Amy Stewart is the author of 10 books, including “Miss Kopp Just Won’t Quit,” the latest in the Kopp Sisters series.

A Maggie Hope Mystery

By Susan Elia MacNeal

Random House. 320 pp. $26