***** S08E01 Benefit of the Doubt (Part One)
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Frost is pulled in different directions, as a man is impersonating him and sexually assaulting women; a young woman, Anne (Joanne Froggatt), has turned up believing Frost is her father; and he has been assigned a new officer, DS Terry Reid (Robert Glenister), who has been abruptly transferred after a complaint from the influential father of a university student, Tim Hamilton (David Birkin), who Reid suspects is selling contraband cigarettes. Frost and Reid's first case involves a woman, identified as Sylvia Carter, whose remains are found dismembered on the railway tracks after being run over by several trains. She worked for a local sandwich maker, and Frost is convinced she was having an affair with someone and was illicitly selling excess sandwich stock for profit.

Meanwhile, Dr Helena Gibson (Arkie Whiteley), a senior surgeon at the local hospital in Denton, goes missing over a weekend. A professional committed to securing proper funding for her department from her cost-cutting boss, Jameson (David Horovitch), her personality left her disliked by many at the hospital, leaving no shortage of suspects.

***** S08E02 Benefit of the Doubt (Part Two)
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Frost solves the Sylvia Carter case after learning she was indeed accidentally hit by a train while crossing the tracks with her affair partner, who hid his involvement out of fear of his wife taking his children.

Reid is seemingly brutally beaten while investigating Hamilton's protection racket, which physically abuses shopkeepers to enforce the sale of illegal cigarettes. When Superintendent Mullett and the press liaison officer accuse Reid of being beaten while buying drugs to feed his old addiction, Frost robustly defends Reid. Frost dedicates himself to cracking the cigarette ring to avenge Reid's assault, and finding Reid's stolen war medal. He traces the medal to teenagers, who can place Frost's impersonator at the scene. Frost arrests the impersonator-identified as one of Frost's former arrests, Larry Brewer-and secures the case against Hamilton. Reid makes a full recovery, admitting he actually slipped while trying to arrest Brewer and hit his head on a sink.

Frost's instincts allow him to connect an abandoned truck of medical waste with the disappearance of Helena Gibson and he recovers her remains before they can be incinerated at a specialist warehouse. There are three prime suspects: Jameson, whom she was going to expose for corruption if he did not improve her department funding; Dr. Retnik (Frank Kovacs), whom she wanted to resign because she believed he was responsible for the high number of unexplained deaths in Bed 5 of the ward; and James Ingram (Bill Fellows), the father of a young boy who died following surgery overseen by Gibson. All the men have unassailable alibis for the day of her death. Frost deduces that Gibson was actually killed days earlier and preserved in the hospital's freezer because their own incinerator was malfunctioning. Retnik is revealed as the murderer. Gibson believed Retnik's capabilities may have been compromised, resulting in the deaths, but had decided to give him a chance. However, Retnik misinterpreted a discussion between Gibson and Jameson as an attempt to oust him, and worried he would lose everything, Retnik killed her. Retnik and Frost fight by the incinerator until Frost gets the upper hand and arrests him. Frost also realises that the deaths in Bed 5 were caused by a cleaner unwittingly unplugging medical equipment for cleaning equipment, ending the inexplicable deaths.

To Frost's dismay, a DNA test reveals Anne is not his daughter, but they part amicably.