Leeds police constable Jo Gillespie (Sheridan Smith) is devastated when her husband, undercover officer Ryan (Kenny Doughty), is killed in suspicious circumstances. As she battles to stay strong for the benefit of daughter Melly (Honor Kneafsey) and stepson Hal (Oliver Woollford), Jo is urged by her bosses, DCI Will Hepburn (Douglas Henshall) and Chief Constable Carolyn Jarecki (Geraldine James), to leave it to her fellow officers to find the killer. But when the murder enquiry starts to uncover some dangerous secrets about Ryan, Jo's faith in the police family of which she has been a part for so long 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), but as they close in on the identity of Ryan's killer, Jo's hunt for the truth will put her own life in danger.