***** S07E01 Line of Fire (Part One)
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As Christmas Day approaches, DI Frost and Dorridge are called to investigate the murder of a young regional crime squad detective, Tim Fox. Fox's leader, Detective Superintendent Finlay and his number two, DI Mark Newcombe, take command of the case, believing Fox was murdered by an organised crime gang dealing in stolen cars. Frost convinces the chief constable to let him rejoin the case to ensure there are no questions of police corruption due to œ1,000 of cash found in Fox's car. Frost believes the killer knew Fox as there are no signs of self defense. During the regional squad's raid on the car thieves, they find the gun that killed Fox. Frost remains unconvinced, especially after learning that Newcombe was having an affair with Fox's wife, Helen. She tells Frost that she had ended things with Newcombe because he had become obsessive and violent, and believes he killed her husband so she would be free to be with him. Frost deduces that Newcombe planted the gun during the raid to frame the gang. He confronts Newcombe, noticing he has a new mountainbike with tires matching those found near Fox's body, but he claims he bought two bikes, gifting one to Helen.

Simultaneously, Frost and Dorridge investigate a
Seriesof pets killed by a rifle, and home burglaries where little is stolen but excessive damage is caused. Elsewhere, a power plant worker, Cockcroft, falls to his death in a mysterious accident after his house was earlier burgled and his fish tank destroyed. Aggressive and misogynistic employee, Ray English, had earlier been told to stay away from Cockcroft's daughter, Tracy.

While attending hospital for an injury, Frost runs into Shirley Fisher, who asks for his help with a 16-year-old who has just given birth and who she believes plans to run away. Frost identifies the girl as a missing person and summons her parents, but advises the girl's father that he must do better if his daughter was too scared to reveal her pregnancy; the father embraces his grandchild and begs his daughter's forgiveness. Frost and Shirley agree to meet later, but Frost is caught in a bank robbery.

***** S07E02 Line of Fire (Part Two)
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Frost convinces the bank robber to surrender. After learning of his predicament, Shirley forgives Frost for missing their date and they agree to spend New Years Eve together. During interrogation, Newcombe tells Frost that he and Helen had planted money to make him appear corrupt so they could threaten him with blackmail, and that Helen killed the violent Fox in self-defense. Frost brings Helen in for questioning and notes that her answers are flawless, making them seem rehearsed. While speaking to his colleague, Ernie Trigg, Frost learns that Helen was involved in record keeping at the station months earlier during a gun amnesty, and that a record of one gun has been deleted.

Meanwhile, Ray and his like-minded friend, Sam Goodwin, attend Cockcroft's funeral, where Sam asks out his colleague Anne, however, Anne's friend, Sally Wainwright, warns her off. In response, Sam shoots Sally's dog dead. Ray takes his own revenge on Sally for rejecting him, by cutting her car brakes, causing a crash that leaves her in a coma.

On New Years Eve, ambitious WPC Susan Kavanaugh investigates owners of rifles that can fire the .22 bullets linked to the pet crimes. She confronts Sam and Ray knocks her out and takes her captive after learning that Sam killed Sally's dog and that they will now be linked to the other crimes. Ray goes to the power plant to create an alibi while Frost, having deduced that the burglaries and shootings are linked to the power plant, goes in search of Kavanaugh, believing she is in danger. He catches Sam attempting to flee and then goes to the power plant to confront Ray who, while escaping, falls through a tall gantry. Despite Frost's efforts to save him, Ray falls to his death.

On New Years Day, Frost informs Ray's ex-policeman father, George, about Ray's death. George, ashamed of his son's actions, claims he has no son, but Frost reprimands him, saying that if George had paid attention sooner, he could have helped Ray and avoided this outcome. Having recovered Helen's bike, Frost arrests her, accusing Helen of killing Fox in cold blood and then letting Newcombe take all the blame.

Frost later celebrates the new year with Shirley, but she is unexpectedly called to work at the hospital; Frost enjoys his indian meal alone.