On a chill November pre-dawn morning a sniper's bullet finds its mark-the head of a homeless woman. Mavis is the third victim in as many days. Small town detectives, Tony Petrocelli and Michelle Baxter are called to the scene by a patrol officer. Like the other killings in the previous days no one heard the shot. One homeless camper's dog alerted to a distant tree before Mavis was killed. A search of the area confirmed someone had used a large branch as a sniper's perch, but no casing for the fatal round was located. Autopsies of the three reveal one of the victims, Jason, was a Vietnam vet who had his leg repaired at Letterman hospital. But Jason changed his last name after leaving the military. What caused him to be homeless is a mystery. A bigger mystery for Petrocelli and Baxter is who is the invisible killer, and what motivates him to kill. The search for answers that can't be found threaten to make this case unsolved-unless the detectives find new evidence.