It was Danielle and Gracie's secret. A teenage adventure. A 1,000 mile drive along the spine of the Rocky Mountains to visit Danielle's boyfriend in Montana. Their parents were never to know. But now the girls have simply vanished. The only person who knows they're missing is Danielle's boyfriend. He persuades his father – a disgraced, suspended cop – to search for them. But he too simply disappears. Now it's up to rookie cop, war widow and single mother Cassie Dewell to find them. Her investigation will introduce her to FBI's Highway Serial Killer Task Force, compel her to confront a spate of roadside sexual mutilations and murders, and lure her towards a darkness greater than anything she could ever have imagined.