After a night out with friends, 22-year-old Sian O'Callaghan disappeared. When her worried boyfriend reports her missing, Detective Superintendent Steve Fulcher (Martin Freeman) begins to piece together her last movements. Believing Sian may still be alive and being held somewhere against her will, Fulcher initiates an urgent full-scale search operation. When the investigation takes a completely unforeseen turn, Fulcher is faced with a life-changing choice between following police protocol and catching a killer.