***** S06E01 Appendix Man
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Several months after DI Frost submitted his resignation, Superintendent Mullett has instead placed him on extended compassionate leave, partly because Mullett is in the running to become the new Assistant Chief Constable, and losing Frost could damage his prospects. However, Frost is forced back into active duty when the press questions a wrongful conviction in one of his old cases, the "Black Farm Murders," which also involved the unsolved theft of valuable paintings: Frost concludes the man claiming responsibility for the murders is just seeking attention. At the same time, Hazel Wallace (Caroline Harker) returns to Denton as a Detective Sergeant and is assigned to investigate the suspicious death of art collector Lester Bryce-Jones. She teams up with Frost after it is discovered that Lester had a Hockney painting-one of the stolen works-which has been stolen again. Fingerprints found at the scene match those of "The Appendix Man", an unidentified individual whose body was discovered in a river a year earlier (first seen in the Series3 episode "Dead Male One".

Frost and Wallace uncover that Lester had multiple aliases and worked as a low-wage hospital porter while secretly selling high-value paintings. They learn that Lester's neighbor, Debbie Viner, had helped him sell the paintings because she needed the money, but he had recently cut her out of the arrangement. When Debbie saw the Hockney painting in his apartment, she went to confront him and found him barely conscious after a risky sexual practice involving alcohol and painkillers. Instead of helping, she let him die and took the painting, only to later discover that it was a forgery.

Frost then deduces that the Appendix Man's fingerprints had been tampered with, and they actually belong to Diana, a pathology lab assistant. Diana reveals that the Appendix Man was Ian Priest, a thief and blackmailer who knew both Lester and her brother, David. All three were gay men, and Priest had been extorting David. When Priest's body came to the lab, Diana altered the fingerprints to protect her brother's secret and his career. Frost concludes that Priest had received the stolen Black Farm paintings but, while in the hospital for appendicitis, feared being linked to the case. Lester, working as a hospital porter, helped Priest leave before his surgery, but when Priest collapsed, Lester threw him off a bridge into the river where his body was found.

Despite solving three cases, Mullett does not receive the promotion, as senior officers discover that Frost had signed his own expense receipts, which Mullett had unwittingly approved. Frost again offers his resignation, but Mullett declines. Frost returns to his marital home, which has finally been repaired.

***** S06E02 One Man's Meat
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When DI Frost's tenant, PC Tom Haliday is injured, DI Frost is left in charge of his police dog, Kaiser. Frost investigates the suspicious death of Jane Owens, a homeless 16-year-old girl found drowned in the Denton River. Frost discovers that Jane, out of desperation, had considered turning to prostitution but changed her mind and rejected her first client. Angered, the client, identified as married father Alan Deering, brutally beat her. Jane's friend Rickie, who was in love with her, leads Frost to Deering, hoping he will be punished, but admits that Jane, distressed by the assault, her dire situation, and grieving a deceased friend, ultimately took her own life. Without Jane's testimony, Frost is unable to prove Deering beat her and is forced to release him, but vows to pursue kerb crawling charges and ensure the case is covered in the local press to publicly shame him.

Frost's second case involves the disappearance of Warren Barber, an environmental health officer who was investigating a local meat processing factory owned by Gerry Ryan. Initially dismissive of the case, the police are pressured to take action by Barber's wife, Naomi. The investigation escalates when a severed arm washes ashore and is identified as Barber's, leading Frost to suspect Barber had uncovered something suspicious at the factory. Frost sneaks into the plant at night and overhears a conversation between Ryan and his accomplice, Jim Scott, revealing that the factory is a front for smuggling drugs from Amsterdam. Barber had broken into the plant after becoming suspicious, but was discovered by Scott, who killed him and disposed of his body by dropping the dismembered parts into the ocean from his Cessna aircraft. Unaware of the murder and feeling remorseful, Ryan threatens to go to the police, prompting Scott to attack him. Frost intervenes, getting wounded in the process, but Kaiser subdues Scott until the police arrive and arrest the ringleaders.

***** S06E03 Private Lives
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DI Frost is partnered with DS Billy "Razor" Sharpe, who is temporarily filling in for the ill George Toolan. They are called to investigate a hit-and-run in a small village that leaves charity worker Bryony Darrow in a coma, eventually leading to her death. Frost initially suspects Owen Kimble (Ross Boatman), a retired soldier with post-traumatic stress disorder who often roams the forest where Bryony was found. As Frost digs deeper into Bryony's financial transactions, he becomes suspicious that she was involved in sex work. This is confirmed by a prostitute informant, who reveals that Bryony maintained a flat in London. When Frost visits the flat, he meets Bryony's mother, Marion, who reveals that Bryony's real name was Natalie Williams. Marion, unaware of Bryony's double life, had been living in the flat without asking questions and had no idea about her granddaughter, Rachel (Paloma Baeza).

Frost surmises that Bryony's charity work was a front for her sex work. A witness links her last sighting to the car of solicitor Graham Rutherford (Tom Chadbon). When confronted, Rutherford admits that Bryony had been arranging a trust fund for Rachel without her husband Richard's (Peter Egan) knowledge. He also reveals that Rachel is not Richard's biological daughter, though Richard and Bryony had agreed to tell her when she was older. Frost deduces that Rutherford was also one of Bryony's clients. A DNA test confirms that Richard was the last person to have had sex with Bryony, and he eventually confesses. Richard explains that Bryony had left her past behind when they started their relationship, but after finding her with Rutherford, they argued. She told him she planned to leave him and take Rachel. Unable to cope with the betrayal, Richard struck her with his car. After learning about her mother's past and that Richard is not her father, Rachel decides to move to London to meet her grandmother.

Meanwhile, Frost and Sharpe are also investigating an armed robbery at a brewery that went wrong because the safe was empty. Frost suspects career criminal Leo Armfield and his accomplice, Garmon, but lacks sufficient evidence to arrest them. When police archivist Ernie Trigg (Arthur White) identifies a pattern in similar robberies, where criminals made a second attempt after failing the first time, Frost sets a trap at the brewery. The plan works, and Armfield and Garmon are caught in the act.

***** S06E04 Keys to the Car
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The newly promoted Assistant Chief Constable, Ms. Cremond, takes charge of Denton station while Superintendent Mullett is on leave. She questions DI Frost's methods after he conducts an off-the-record interview with drug dealer Richie Dearne to coerce a confession related to a man left comatose from drugs supplied by Dearne. Dearne's solicitor, Simon Warrinder, files a complaint, leading Cremond to release Dearne and order Frost to cease further contact. Meanwhile, Mullett reports the theft of his golfing partner Ben Pecksmith's red Mercedes from their golf club by a suspect named Harry Plummer, a temporary club member. Plummer checks into a luxury hotel using Pecksmith's identity, where he meets a Dutch woman, Marijka Hoogenbloom. After abandoning the Mercedes, Plummer steals a Range Rover and travels to Holland with Marijka.

The abandoned Mercedes is traced to the hotel. When Frost investigates, he discovers Dearne's bludgeoned body in the car's boot along with œ5,000 in cash, launching a murder investigation. Marijka is arrested upon her return to the UK, and Plummer (real name Alan Carpenter) is apprehended as well. Frost determines that Carpenter is merely a thief involved in smuggling pornography tapes from Holland and had no involvement in the murder. Marijka, on the other hand, is revealed to be an undercover Dutch drug officer who was at the hotel to meet the real Pecksmith, suspecting his biomedical company of smuggling drugs from Holland. As the investigation progresses, Frost realises that the comatose man's girlfriend, Philly Williams-Pecksmith's stepdaughter-had contacted Dearne in desperation due to her addiction. She fled after seeing her stepdad's stolen Mercedes at the meet-up location in Fulford Woods, only to spot a blue car arriving. Frost deduces that Warrinder met Dearne in Fulford Woods at the same time Carpenter was there for a sexual liaison.

Frost races to the courthouse, where Warrinder is being held hostage by his client Luke Hands, whom Frost had previously interrogated in connection with Dearne's death. Fearing he will be blamed for the murder, Luke forces Warrinder to confess at gunpoint. Warrinder admits that Dearne had been blackmailing him over his drug addiction and demanded the œ5,000, so he killed him and hid his body in the Mercedes. Frost calms Luke down, leading to Warrinder's arrest. Warrinder insists his coerced confession will not hold up in court, but Frost reveals that both Carpenter and Philly can identify him. Although Cremond reprimands Frost for confronting a gunman without a protective vest, she acknowledges his success, though Frost claims the gun was unloaded and there was no danger. Frost later privately reveals he had removed the bullet from the gun after Luke surrendered it.