Catherine Cawood is a policewoman in the ironically named Happy Valley, the nickname for 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.
Learning that Nevison's wife Helen is terminally ill Kevin feels increasingly guilty and tells his own wife Jenny what has happened. She advises him to deny all contact with the others and refuse to accept any money so that when, on Nevison's instructions, he takes the ransom to Cowgill, he hides his own share. Lewis is concerned that Royce is ill-treating Ann and plans to kill her when the full ransom is received but is savagely beaten by Royce when he tries to intervene on her behalf. Catherine meanwhile starts a poster campaign to locate Royce and, after arresting local councillor Marcus Gascoigne for drunk driving and arguing with Richard for refusing to accept Ryan as his grandson, she gets a call leading her to the house where Ann is being held. Royce hides when she rings the bell but becomes paranoid as to how she knew where to look for him.
Following Catherine's visit to the house Royce moves Ann but the van in which he and Lewis are taking her is stopped for speeding by policewoman Kirsten McAskill and so Royce, to Lewis's horror, runs Kirsten over, killing her. Reminded of her daughter's death and accused by Kirsten's partner of pushing her into action Catherine is devastated and, whilst Richard offers to accept Ryan, she can do without being told not to prosecute Gascoigne for drink and drugs as he donates to the police fund. Cowgill is also appalled by Royce's actions, particularly as Royce wants to kill Ann to stop her talking but gives Nevison some hope by suggesting Ann will be released after another ransom payment delivered by Kevin. As a result Helen, who wanted to speak to Catherine, tells her she has changed her mind. Delivering the money to Cowgill Kevin is shocked when Lewis tells him that Royce killed the policewoman whilst Catherine decides to investigate the house where Royce was seen and is suspicious of what she finds in the cellar.
Catherine is pleased that Richard is bonding with Ryan. She also learns that the house she investigated belongs to Cowgill's wife. She visits both the wife and Cowgill himself, though he feigns ignorance of any illegal activities. She also calls on Royce's mother Lynn who indicates she has heard that Royce is Ryan's father. Another visit to Lynn leads to a life-threatening situation.
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.
Clare organizes a birthday party for Catherine with the family, colleagues and the Gallaghers but makes the mistake of telling Daniel that Catherine has been seeing Richard. At the party Daniel gets drunk and accuses his parents as well as harshly criticizing Becky, whom he claims was a spoilt bitch and Royce's willing partner, rather than the victim they have all beatified. Next day Lewis's corpse is discovered and Cowgill is murdered by the drugs gang. Royce hides out on a narrow boat and contacts Ryan, explaining that he is his father and encouraging him to visit on the boat. Catherine finds out and races to save her grandson, leading to a confrontation with Royce before the family is reconciled.
Eighteen months have elapsed. While investigating a bizarre case of sheep-rustling, Catherine discovers the corpse of prostitute Lynne Dewhurst, Royce's mother, whom she had warned to keep away from her grandson. Along with new recruit Ann Gallagher, Catherine joins the investigation. The woman has been killed and sexually assaulted. CID officer John Wadsworth, meanwhile, is more concerned with preventing his mistress, Vicky Fleming, from telling his wife Amanda about their affair, whilst Ann's father Nevison has to sack volatile employee Sean Balmforth, and Clare meets an old flame, Neil Ackroyd. Royce is given consolation in prison by besotted visitor Frances Drummond, to whom he suggests that Catherine killed his mother.
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.
Catherine reconciles with Clare but her superiors see her as unstable and offer medical leave, which she rejects - accepting to see a therapist instead. Vicky's body is discovered but the murder of yet another prostitute allows John to suggest that both were the serial killer's victims. His marriage is on the rocks anyway. Catherine is shaken by the apparent suicide of one of Ilinka's slave-masters, whom she had tasered and arrested but who had been released on bail. Ilinka thinks he has been silenced. Meanwhile, deceived by Royce's lies, vulnerable, besotted Frances Drummond is encouraged to ruin Catherine's life.
Catherine interviews a prostitute who has survived being attacked by Sean Balmforth, who is subsequently arrested and charged as being the serial killer whilst vulnerable young farmer Daryl Garrs is also taken in for hitting out at the thugs who stole his mother's sheep. Catherine is shocked when, on grandson Ryan's birthday, he receives a lavish present, purportedly from his father Royce, which allows Drummond to suggest to the boy that he is anxious for forgiveness. In fact Royce wants a more direct revenge. As John's wife throws him out Neil tells Clare that he once had an affair with Vicky Fleming and believes her murder to be different from the others.
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.