In the early hours of a quiet, weekend morning in Manhattan's Diamond District, a brutal triple murder shocks the city. Lincoln Rhyme and Amelia Sachs quickly take the case. As more crimes follow, it becomes clear that the killer's target is not gems, but engaged couples themselves. The Promisor vows to take the lives of men and women during their most precious moments --- midway through the purchase of an engagement ring, after a meeting with a wedding planner, trying on the perfect gown for a day that will never come. Soon the Promisor makes a dangerous mistake: leaving behind an innocent witness, Vimal Lahori, a talented young diamond cutter who can help Rhyme and Sachs blow the lid off the case.