The following version of the book was used to create this study guide: Raybourn, Deanna. Killers of a Certain Age. Penguin Random House, New York, NY. 2022. AZW file.

Sometime in the late 1970s, a young woman named Billie Webster is arrested during a protest. At the police station, a dapper Englishman named Richard Halliday offers to get Billie out of her situation if she agrees to join his organization. Dubbed the Museum, this is an extra-governmental agency that is overseen by a three-member Board of Directors. The Museum hires and trains assassins, originally targeting the Nazis who had escaped Germany following World War II. By the 1970s, there are few Nazis remaining, the directors turn their attention to other evil people such as sex traffickers, dictators, and drug smugglers. Billie joins a four-woman team called Project Phoenix. Together with Mary Alice, Natalie, and Helen, Billie learns everything from languages to hand-to-hand combat from their trainer Constance Halliday. The Sphinxes soon carry out a series of successful missions. They pose as nuns, stewardesses, or nurses and learn all the nuances of those roles in order to be most effective.

As the Sphinxes reach 60 years old, they are invited to a retirement party aboard an elegant cruise ship. They are older and have adopted some of the normal life situations for women of their age. Helen is a widow, and Billie and the others worry about the level of grief she continues to display. Mary Alice is married to a woman named Akiko. Mary Alice has kept her true job hidden. During their first night on the cruise, Billie steps into a service room to take advantage of a walk-in cooler to deal with a hot flash. She sees another Museum employee – Brad Fogerty – and is on the verge of announcing herself when she realizes that he apparently does not want them to acknowledge his presence. Billie and Helen search Brad's room and find a bomb set to go off in a relatively short amount of time. With few options, Billie kills Brad and plants the bomb in the ship's engine room. They make a small fire to clear the ship, slipping away in a small boat while everyone else boards lifeboats.

The four women have limited resources. However, by using those they have and their wits, they make their way to New Orleans where Billie owns a former convent. Though run-down, it serves as a safe house. They make contact with Sweeny, one of the men who also worked for the Museum. They believe he will be an ally, but Sweeny is clearly planning to kill the four women to collect the bonus promised by the Museum. The four Sphinxes – joined by Billie's friend Minka and Mary Alice's wife Akiko – realize the Museum's directors have ordered their deaths as a means to cover up their own misdeeds. They begin working through those top officials along with a few others, including bodyguards. There is one second-in-command remaining – a woman named Naomi Ndiaye – who agrees that the Museum's directors have lost focus of their original mission. She helps the Sphinxes in the final stages, comes to an agreement that includes calling off those who are trying to kill them, and pledges to get the Museum back on track. The four women set out to resume their lives. On the final page of the story, the author warns that she has changed enough details that the reader “can't find us, even if you wanted to” (349).