Nurse Hester Latterly finds herself well suited for the task: accompany Mrs. Mary Farraline, an elderly Scottish lady in delicate health, on a short train trip to London. Yet Hester's simple job takes a grave turn when Mrs. Farraline dies during the night. And when a postmortem examination of the body reveals a lethal dose of medicine, Hester is charged with murder-punishable by execution.

This notorious case presents detective William Monk with a daunting task: find a calculating killer among the prominent and coolly unassailable Farraline clan-and try to save Hester from the gallows.