A psychologist monitors his own pulse while reading and is drawn into a mystery when a distressed young man delivers a frantic letter and a photograph of his fiancée. A cryptic classified advertisement and the visit of a coarse, hammer‑wielding stranger have convinced the woman to abandon their engagement and adopt a deathlike detachment; the advertisement's coded numbers and repeated publications suggest an unseen campaign. The woman's guarded past, facility with several languages, and obscure origins deepen the puzzle. The psychologist applies scientific observation and investigation to unravel a scheme of psychological coercion and anonymous messages that may threaten more than one life.