Lincoln Rhyme and Amelia Sachs take on the investigation of a brutal triple murder in Manhattan's Diamond District. The killer, known as the Promisor, is murdering men and women midway through the purchase of an engangement ring.

A businessman snatched from an Upper East Side street in broad daylight. A miniature hangman's noose left at the scene. A nine-year-old girl, the only witness to the crime. With a crime scene this puzzling, forensic expertise of the highest order is absolutely essential. Lincoln Rhyme and Amelia Sachs are called in to investigate. Soon the case takes a stranger turn: a recording surfaces of the victim being slowly hanged, his desperate gasps the backdrop...