***** S09E01 Mistaken Identity (Part One)
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While Superintendent Mullett and the Denton station prepares for the arrival of Babcock (Michael Cochrane), from Her Majesty's Inspectorate of Constabulary (HMIC), which will determine their future funding, Frost and a new CID team member, DC Ronnie Lonnegan (Michelle Joseph), investigate three murders: an unidentified body found in a reservoir, and the deaths of Jean and Paul Harris, who were bound, doused in petrol, and burned alive. The main suspect is the Harris's handyman, Mike Patterson (Jason Merrells), who was having an affair with Jean, as she was unsatisfied because of Paul's paraplegy, and Paul's father Arnold Harris (Barrie Ingham), who disliked Jean.

Forensics indicate a connection between the drowned man and the Harrises' murder owing to the presence of garden lime at both crime scenes. Frost recruits psychologist Pam Hartley (Susan Penhaligon), on the Harris case, who surmises he is pursuing a true psychopath, who enjoyed watching them burn. .Frost cares for a dog after his homeless owner is beaten by a mysterious suspect who set fire to a car owned by Paul Harris's father

Frost also intervenes in an ice cream van turf war after discovering one of the companies is dealing ecstasy alongside sweets, and investigates the disappearance of a young woman, Melanie Monkton (Kate Maberly), after her beloved horse is euthanised following a break-in.

***** S09E02 Mistaken Identity (Part Two)
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Frost meets Melanie's wealthy and self-absorbed parents, Harry and Fiona (Annette Ekblom), who are not overly concerned about her disappearance, even after her car is found abandoned. Melanie eventually returns, revealing she spent the weekend in London with Paul Matthews, a jockey with whom Fiona was having an affair, and Fiona arranges an assault on Paul for seducing her daughter. The investigation also reveals that Harry was committing insurance fraud by faking the euthanisation of Melanie's horse to help settle his debts.

Frost interrogates Mike Patterson but determines he is not the serial killer. The drowned man is identified as Reginald Molloy, and Frost realises that Harris's father and Molloy's mother were both witnesses 13 years earlier during the trial of brutal thug Charlie Layman, for a fatal armed robbery. However, Layman died a year previously. After meeting other witnesses, Toolan and Lonnegan learn that they have also had relatives die over the preceding year, including a young child. It is also learned that the families had all used a similar looking man for handywork, indicating the killer got close to each of them before striking. Frost and Toolan deduce that the killer is targeting the witnesses' loved ones to make them suffer.

Following another murder, Frost deduces that the killer is Layman's son, Jeffrey Meadows (Jonjo O'Neill). He had been put into care as a child to protect him from his abusive father and drug addict mother, but his father continued to send him letters to control him from prison. Pam was the psychologist who recommended he go into care. Jeffrey abducts Pam and traps her in the pipes of an auxiliary pumping house, which he floods with water. Frost and Lonnegan track down Jeffrey, but with his work complete, he commits suicide by leaping from a bridge. Mullett is able to surmise where Pam may be and Frost rescues her just before she drowns.

The HMI, who has a vendetta against Frost for his unorthodox methods, proposes withholding the Denton stations funding, but Frost and Mullett are able to convince him that the serious crimes they have just solved prove the station's worth.