Scotland Yard Inspector Michael Pretherston has been called to attend a case of missing jewelry at Lord Flanborough’s home. While investigating, Inspector Pretherston comes across a familiar face—and an intriguing mystery to challenge his skills and intellect: a woman by the name of Katharine Westhanger.

Kate has been a person of interest in various capers across Europe despite her innocent looks and charming personality. The use of a false identity is unlikely to be an attempt to distance herself from the tainted reputation of the Westhanger name, but what lies behind her latest appearance in London and recent association with the rich and powerful?

Inspector Pretherston is certain that the Crime Street gang is scheming again, and intends to find out what their next target could be. Meanwhile, Kate discovers new thrills in her multiple encounters with Michael, playing a mischievous game of cat and mouse that mixes crime, romance, and a battle of wits and opposing personalities.

Published in 1917, Kate Plus 10 was adapted into a motion picture of the same name in 1938, and later became the basis for the film The Trygon Factor .