DCI Gabriel Markham and his team are working to find a serial killer when a break in the case leads them to a potential culprit. Gabriel is torn between his conviction that they already know who the killer is and the seemingly irrefutable evidence found at the last murder scene.

Gabriel and the team are dealing with the fact that another body has been discovered, seemingly the work of the serial killer. Questions are raised about Gabriel’s suitability to be leading this case, and the rest of his team have to step up to the plate when he's called away on a personal mission - one that strikes at the heart of his family.

An unexpected tip off convinces Gabriel and his team to launch a risky mission designed to catch the killer in the act. Meanwhile, Elaine confronts the demons of her past. Gabriel is forced to face up to his changed self and the indelible mark left on him by the killer.

The story of the acid murderer's first victim is revealed, as another suspect enters the frame. Gabriel receives some help in his investigation from an unexpected source - but is the situation more complex than it seems?

Following Gabriel's discovery of a crucial piece of evidence, the hunt is on to track down the killer. With the net tightening, a thrilling chase of twists and turns ensues, and as Gabriel begins to piece together the trail of violence leading to the killer, he ends up putting everything he holds dear at risk.