Police constable Jo Gillespie (Sheridan Smith, Jonathan Creek) is devastated when her husband, undercover officer Ryan (Kenny Doughty, Vera), is killed under suspicious circumstances. As she battles to stay strong for the benefit of her daughter and stepson, Jo is urged by her bosses, DCS Will Hepburn (Douglas Henshall, Shetland) and Chief Constable Carolyn Jarecki (Geraldine James, Sherlock Holmes), to leave it to her fellow officers to find the killer. But when the murder inquiry starts to uncover some dangerous secrets about Ryan, Jo's faith in her police family is severely tested. No longer sure who to trust, Jo embarks on her own investigation with the help of friend and colleague Jack Clark (Matthew McNulty, The Paradise). But as they close in on Ryan's killer, Jo's hunt for the truth will put her own life in danger. Written by Matt Charman (Bridge of Spies) and directed by Michael Samuels (Any Human Heart), this powerful crime thriller dives into the murky depths of undercover police work and tells the story of a woman willing to risk everything to protect her family.