Severely injured on a case, Page retired from the FBI to become a professor. But now his friend has been killed by a sniper in New York and Page's extraordinary mind is needed urgently. The shot that killed the man should have been impossible: in the middle of a blizzard, down a busy New York avenue, into a moving car. But it happened.

Only Page can work out the science behind the shot, the geometry that reveals the killer's location. The logic that says the shooter has killed like this before. And will do it again, and again, until they are stopped ...