When a sniper kills four people on Boston Common, Boston homicide detective Mike Houston and his partner Anne Bouchard are sent to investigate the case. Amidst the blood and terror, Houston discovers similarities, likenesses—the killer's positioning, his choice of victims, and his code of ethics—between the crime scene and his own training as a US Marine scout and sniper. And with the staging of the scene set for prime shock value, Houston has to wonder what it is this murderer intends to accomplish.
The connection is confirmed in the worst possible way when the sniper strikes again, this time killing Houston's ex-wife, severing what's left of the bond between Houston and his estranged daughter, Susie. It's personal now, and as the death toll rises, Houston and Bouchard will stop at nothing to find the cold-blooded sniper who's making a mockery of their department.