One is chasing justice. One is chasing the past. Both are being followed.

Deep beneath London, urban explorer Connie Darke stumbles across a mutilated body in the city’s decaying sewer system. Moments later, a strange whistling sound follows her through the tunnels—low, eerie, impossible to trace.

DI Lew Kirby is assigned the case. The victim is a wealthy financier with no clear reason to be underground—and no shortage of enemies above it. As Kirby investigates, Connie uncovers chilling new clues about the unexplained death of her sister. Both are drawn deeper into a world of hidden networks, buried secrets, and obsession.

And someone out there is watching them both, following them both…

As the city’s hidden layers begin to crack open, Kirby and Connie must confront the terrifying possibility that whoever, or whatever, is stalking them has been waiting a long time to emerge.