Lady Frances Ffolkes vows to unmask a mysterious assailant threatening her friends, only to find herself embroiled in a murder investigation at a remote estate. Lady Frances Ffolkes travels to the fabled Kestrel's Eyrie estate after her friends Gwendolyn and Thomasina receive menacing threats. Her investigation into the attacker takes a grim turn when Gwendolyn's father, a prominent diplomat, is discovered murdered with his own ruby dagger. Frances must navigate the secrets of the estate and its inhabitants to find the killer before they strike again.

The premise sets up a classic country house mystery with a determined amateur sleuth. The focus on interpersonal relationships and hidden motives within an aristocratic setting suggests a narrative rich in social observation and deduction. The inclusion of a murder with a unique weapon, the ruby dagger, points to a plot designed to intrigue and challenge the reader's assumptions. The resolution likely hinges on piecing together clues and understanding the complex dynamics of the estate's inhabitants.