1987: One morning, fifteen year old Tania Mills kisses her mother, walks out of her front door and disappears. 2014: When a new lead comes in for Tania's case, DS Sarah Collins is asked to investigate. PC Lizzie Griffiths, the subject of Collins' last investigation, is back on the job and working at a domestic violence unit. When a quirk of fate brings Lizzie and Collins back into contact, each must try to reconcile the claims of the past with the demands of the present.