Set in Cheshire in the small fictional town of Marshwell, detectives Michael Niles (Neil Stuke), Nina Suresh (Indira Varma), Alec Wayfield (Dino Fetscher) and Bobby Day (Robert Glenister) begin to piece together what happened on the fateful morning Doctor Angela Benton took her three year old son Luke to the playground.
Watching Luke play alongside other Mums and their toddlers, Angela is set upon and brutally stabbed by a man in a hoody whose face is covered from view. His jerking head movements and disturbingly weird sounds became apparent moments before the attack, and eyewitness and local Garden Centre owner Lucy Cannonbury (Lesley Sharp) is keen to help the detectives. With Lucy's version of events instantly ringing alarm bells with dedicated, compassionate cop Nina Suresh, fellow seasoned detective Bobby Day, is more willing to accept Lucy's account of Angela's tragic murder.
As the investigation unfolds, Bobby is convinced someone is watching them when he catches sight of shadowy figures. Bobby and his colleagues also begin to receive mysteriously worded notes from the 'Ghost Detective' helping them with insightful details about the case.