In a Glasgow post office one week before Christmas, Martin Pavel cowers on the floor, his eyes locked on those of a terrified child. Above them, a masked gunman wields an AK-47 while the boy's grandfather calmly volunteers to help the robber gather the money. Then the old man stands passively, hands by his sides, as the gunman raises the barrel and shoots him to shreds. Recently returned to work after the birth of her twins, DS Alex Morrow is called in to head the investigation. Why did the grandfather offer his help? Was it a noble act of self-sacrifice, or did the old man and his killer share a dark past?