Catherine Cawood is a policewoman in the sarcastically nicknamed Happy Valley; the drug-affected Calder Valley in West Yorkshire, England. Her private life is complicated. She still sees and sleeps with her ex-husband, journalist Richard Cawood, even though he has a new woman in his life. She lives with her sister Clare, a recovering addict, and grandson, Ryan, whose mother, Catherine's daughter, Becky, killed herself after being raped. Now the man Catherine believes to be the rapist, Tommy Lee Royce, is out of jail and back in town. At the same time, Kevin Weatherill, disgruntled that his boss Nevison Gallagher will not give him a raise so he can enroll his daughter into a good school, conspires with caravan park owner Ashley Cowgill and his employees, Lewis and Royce, to kidnap Nevison's adult daughter, Ann, for a ransom. When Nevison has a change of heart and offers to pay for both his daughters' tuition, Kevin wants to abort the operation but the others go ahead with it. As the kidnapping unfolds, more people are drawn into the investigation and conspiracy, leading to tragic results.

Catherine and Ann manage to escape and are rescued by the police, but Kevin's questionable account of his involvement leads to his arrest and remand in custody and, via his phone calls, the arrest of Cowgill. Cowgill enters into a plea bargain for a reduced sentence by giving information about a vast drugs importation racket, and as a result he and his family will need witness protection. Catherine recovers in hospital and is visited by Ann, who asks her not to tell her mother about her rape by Royce. On discharge, Catherine is despondent since Royce is still at large.

Catherine brings home Croatian Ilinka, one of several women rescued from human traffickers, and places her with elderly Polish friend Winnie before joining the house-to-house investigation of the serial killer who murdered three female victims. After his wife's death, Nevison reinstates Sean but Catherine suspects the young man of stalking prostitutes. She also attends Lynn's funeral, which Royce has been allowed to attend. He makes a scene and accuses her of killing his mother, a fact which does not please her superiors. Also present is Frances Drummond who, under an assumed name, has started teaching at Catherine's grandson Ryan's school. Returning home, Catherine argues with her sister, who has started drinking again, whilst John's attempt to end his affair with Vicky does not end well.

Following a tip-off Catherine makes a gruesome discovery at the Garrs' farm, leading to an arrest, as well as learning Drummond's true identity and confronting her with the facts about Royce - though some damage has already been done. Hearing Neil's account of Vicky Fleming's penchant for blackmailing her married lovers and a statement from John's wife's boyfriend means that the net finally closes in on Vicky's killer, once again involving Catherine in a precarious situation.