***** S05E01 Penny for the Guy
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DI Frost is forced to cancel his holiday plans to investigate the suspicious death of a young boy, Dean Anderton, on Halloween, working alongside the ambitious DS Liz Maud (Susannah Doyle). The autopsy reveals Dean was chloroformed, had a finger surgically removed, and died from choking on his own vomit. His body was dumped, and another child, Bobby Kirkby, has been kidnapped. The following day, a ransom note is delivered to Sir Richard Cordwell, the wealthy CEO of Cordwell Supermarkets. The kidnapper demands œ500,000 from Cordwell, to which he agrees, seeing an opportunity for positive publicity. Cordwell delivers the ransom to a secluded wooded area while Frost and Maud observe. An elderly man, Henry Finch, arrives with his dog and discovers the money bag, but is struck over the head by an unknown assailant who escapes with the money. Frost later learns from an informant that a year earlier, Cordwell Supermarkets had unwittingly accepted œ40,000 in counterfeit notes. Unable to deposit them, Cordwell ordered the staff to distribute the fake bills to customers, swearing them to secrecy. Using this information, Frost tracks down the suspect who used the counterfeit notes to buy a car and arrests him. The suspect, however, has an alibi for Bobby's kidnapping and claims Finch fought him for the money, raising Frost's suspicions.

Frost discovers Finch is terminally ill and may be enacting a plan with limited time left. A search of Finch's home yields no evidence, but Frost learns Finch is house-sitting for friends. When Frost confronts him, Finch denies involvement in the kidnapping but reveals a motive: Cordwell had fired Finch's wife a year earlier when she tried to report the counterfeit money. Distraught, she took her own life. Frost suggests Finch was not after the ransom and only wanted to hurt Cordwell, but needed a live child for the ransom message, and took Bobby when Dean accidentally died. Frost revisits Finch's friend's house and finds a petrol station receipt in Finch's name, leading him to search nearby Denton Mill, where Bobby is found alive. Although Finch's clever plot leaves no direct evidence linking him to the kidnappings, Frost plants the receipt at the scene, allowing the police to charge Finch.

Simultaneously, Frost and Maud investigate the abduction and ransom of teenager Carol Stansfield. It transpires that Carol orchestrated her abduction with her friends to get money from her inattentive father who prioritised her step-mother.

***** S05E02 House Calls
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DI Frost works with DS Liz Maud to investigate incidents where children have been injected with water using a syringe. Frost connects the incidents to Sydney Snell, an intellectually disabled man who believes he is helping the children. Feeling sympathy for Snell, whom he had previously sent to prison with a promise of psychiatric help that never materialised, Frost releases him. Upset by Frost's leniency, Maud reports him for neglecting his duty. Simultaneously, Frost and Maud investigate the discovery of career criminal Lemmy Hoxton's body, dead for three months. They uncover that Lemmy had been posing as a utility worker to commit robberies and was last seen on his way to the country home of Julie and Caroline, who presented themselves as sisters. Frost and Maud deduce that Lemmy attempted to rape the couple and was killed in self-defense. Fearing exposure of their relationship, Julie and Caroline hid the incident.

Meanwhile, carpet fitter Mark Grover returns home from a job at Bonley's department store to find his two infant children dead and his wife, Nancy, missing. Autopsies reveal the children were injected with water after their deaths, making Snell the prime suspect. Superintendent Mullett warns Frost of a likely inquiry into his handling of Snell, and DCI Peters takes over the case. Later, Nancy's body is discovered in a railway tunnel. Though she had a history of depression and suicidal threats, the coroner finds stab wounds that suggest murder, with her body thrown onto a train to stage a suicide. Peters coerces a confession from Snell, who admits to injecting the children but insists they were already dead when he found them, raising Frost's doubts about his involvement. Frost's suspicion shifts to Mark after learning Nancy had recently sought an abortion that was denied due to her mental state. Frost discredits Mark's alibi, discovering that he had been let off Bonley's premises to use leftover carpet remnants in his home. During questioning, a distraught Mark confesses: he and Nancy had argued after he returned home, and Nancy, overwhelmed by depression and her struggles with family life, smothered the children. Enraged, Mark killed her, and his partner, Phil Collard, helped conceal the crime by throwing her off a bridge.

Maud tells Frost she is transferring to another station with better career prospects, but apologises for reporting him and they part amicably.

***** S05E03 True Confessions
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DI Frost investigates after the affluent Jeanette Barr (Yolanda V zquez) commits suicide by jumping from a bridge into a river. However, the autopsy reveals that the water in her lungs means she was drowned in a bathtub and dumped in the river. Frost's main suspects are her husband, James Barr (Anthony Calf), whom Frost investigated a decade earlier following the murder of his previous wife, Harriet, by an intruder, and Richard Sheridan, an employee at Barr's company, with whom Jeanette was having an affair. Frost and Superintendent Mullett worry that, if Barr killed his second wife, they potentially imprisoned the wrong person for Harriet's murder. Sheridan is given an alibi by his infuriated wife, while Frost meets with local priest, Father Sullivan (Eoin McCarthy), who recently heard confession from someone claiming they were going to commit murder.

Frost's case is complicated when he is suspended by Detective Superintendent Bailey (Gwyneth Strong) of the Discipline and Complaints department, who is investigating falsified evidence in historic cases worked by Frost's former chief superintendent, Charlie Fairclough (Kenneth Cope). Frost meets with Fairclough, who admits he planted evidence to secure convictions, including against Harriet's killer. Frost meets with a retired priest who knew James and his brother, Alec, as children: Alec was devoutly religious in his youth, but as his family struggled, Alec became intensely jealous of James, vandalising his possessions, being linked to a pet's death, and obsessing over stealing James's girlfriends.

Frost convinces Fairclough to confess his misdeeds to Bailey to close her investigation and have Frost reinstated so he can pursue Alec. Sullivan helps Frost trick Alec into admitting that he murdered Jeanette because James had gifted her his shares in the family business, which she was threatening to sell to a firm pursuing the company's assets, unless Alec paid an exorbitant sum. She lured Alec to a hotel under the pretence of selling the shares and then seduced him, keeping the receipt as blackmail material. When Frost accuses Alec of killing Harriet out of jealousy of his brother, he flees, running into the road, where he is killed by a passing truck.

***** S05E04 No Other Love
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Superintendent Mullett begins assessing staff for redundancies due to budget cuts at the Denton station, and Frost's job is at risk. Meanwhile, DI Frost moves into a new flat, and allows his old partner, DS Clive Barnard, stay with him while they handle investigations into an armed robbery at Peter Lawson's (Mark Lambert) pawn shop, and a case in which the elderly Olive Walters (Jean Heywood) murdered her husband Harry. Frost learns that Olive's actions were impulsive, the result of decades of emotional abuse from her husband, whom she held responsible for the death of their 2-year-old daughter nearly 40 years earlier, because he refused to let her go to hospital to treat her fever. Olive is released and later dies of natural causes.

Frost's investigation into the robbery uncovers Peter's teenage daughter, Rachel, is being sexually abused, and her brother, Matthew, robbed their father for money to fund travel to their elder sister, Joanna (Luisa Bradshaw-White), in Blackpool where Rachel gets an abortion. Rachel and Joanna, who was also abused, are worried that their youngest sister, Suzanne (Ashlie Walker), will soon be targeted. When Peter is murdered, Frost initially suspects the children, but soon uncovers the truth: Peter's domineering father, Charlie (Anthony Bate), killed him to cover up the fact that he had been raping his own granddaughters. When Frost confronts Charlie, he holds Frost at gunpoint, and Clive is killed while protecting him. Charlie then turns the gun on himself.

Distraught, Frost tenders his resignation from the force and buries his George Cross medal with Clive, declaring him a true hero.