When a strange diary written by what seems to be a tormented, terrified child turns up in a bar in Idaho, detailing the grisly atrocities committed by the diarist's 'master', Special Agent Sylvester thinks he may finally have a lead on his killer. The diary also brings in Tristan Lau, Walker's colleague with benefits and a man with a stubborn case of his own. As their cases prove connected in ways they never could have anticipated, Walker finds himself guiding Lau through a secret world of magic even as their own connection deepens.

Sylvester has long grappled with paranormal crimes as a federal agent, gaining a reputation that brings a strange case to his an artificial angel has tumbled through reality after escaping imprisonment, but the process is tearing his unnatural form apart and he is desperately in need of rescue. Meanwhile, his partner Tristan soon finds that his case of human trafficking is anything but ordinary, dragging him further into Sylvester's supernatural world...