"There's been a police-involved shooting... three officers have been shot, I need Armed Response and ambulance services here immediately!"

In the early morning darkness, an undercover drug meeting goes horribly wrong, leaving two officers dead and a third wounded. Coincidences pile up as evidence of both a planned attack and of an eyewitness comes to light. Convinced that these murders were a planned hit, the investigators begin to explore the pasts of all officers involved, including one of their own...

For Kate Gardener, seeing what, or who, isn't there comes naturally. Finding them is another story...

When Detective Sergeant Richard Pierce's connections to the victims and a shadowy Bosnian drug trafficker put the case at risk, Kate steps up her search for the "watcher from the walkway". Innocent lives are in grave danger, including her own, and the final revelation of who was responsible sends shockwaves through the entire Met.