***** S10E01 Hidden Truth
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London drug lord Jack Flynn murders one of his dealers in Denton. The only witness, Cathy Thompson, is placed in protective custody, but Flynn's professional hitman, George Starkey, assaults her home and critically wounds a WPC protecting her before fleeing. Frost is tasked with protecting Cathy and her young son, Robbie, until she can testify against Flynn, and requests DS Terry Reid's assistance owing to his connections with the Metropolitan Police in London. Flynn hires a second, younger and brasher, hitman, Gary Tinley, to distract the police from Starkey. Armed response team members Alan Hadley and Kenny Russell seemingly spot Tinley, but Russell is killed while pursuing him. Frost deduces that the precision of the gunfire indicates that Hadley actually killed Russell, especially after Tinley is arrested and admits he dropped his gun during an earlier encounter with the police. Reid confirms this after learning that Hadley's wife had been having an affair with Russell. They later find Hadley has hanged himself out of guilt.

Starkey attacks the new police safehouse that only select personnel know about, and though he again flees, Cathy uses the distraction to run away with Robbie, no longer wanting to risk their lives to put away Flynn. Frost realises that someone is leaking information to Flynn, discovering that Eileen Cleary, a member of the prosecution team against Flynn, is responsible, as she had a son with Flynn six years earlier. Cleary is arrested, and while Frost and Reid review Cathy's witness statement, they discover that Robbie witnessed the crime, but Cathy took responsibility to protect him. Frost and Reid race against time, as Cathy had agreed to meet Cleary at the Crannock Air Museum. Starkey attacks Cathy as the police close in, but Frost and Reid successfully capture him. Robbie testifies against Flynn, securing the case against him. Later, the Denton officers finally find Frost's winning lottery ticket worth œ5 million, lost in his typically sloppy file-keeping, but learn that while his numbers are correct, his ticket is out of date.

***** S10E02 Close Encounters
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Frost finds himself working with DS Hazel Wallace again when two separate cases need his attention. The first involves a
Seriesof robberies at newly-bought houses. Frost uncovers a lead when a young boy of a hostile divorced couple goes missing, and the father is unable to explain where he was that night. He deduces that the father, who works for a removals company, has been tipping off burglars in exchange for cash to help with his financial struggles. Frost's arrest of the gang includes a high-speed chase of their scout, flanked by a fleet of delivery bikers from Frost's favourite fast food restaurants, and he locates the missing child hiding in his father's workplace, hoping in vain to reunite his parents in their search for him. Although they do not get back together, they do become more amicable towards each other.

The second case involves a murdered security guard at Lightfoot quarry, a crime that was witnessed by an autistic young man, Laurence Burrell (Jim Sturgess), who believes aliens and the American government are after him. Forensic evidence links Burrell to the murder weapon and he is arrested, but he is unable to cope trapped in an unfamiliar location and absconds. The quarry's owner, Charlie Lightfoot, accuses his old business partner, Stuart Mackintosh, of attacking the quarry and killing the guard to scare Lightfoot into selling it cheaply. Mackintosh needs the quarry, which will serve as a lake in his Denton Park housing development, as his own financial state relies on the project's completion. Lightfoot offers to sell it for an extortionate price as revenge for Mackintosh leading him into financial ruin decades earlier. Burrell returns to the quarry and steals explosives, intending to blow it up so the aliens will leave him alone, and Frost risks his life to try to save him. When Frost is left incapacitated, Burrell is able to calm down and helps move Frost to safety before the explosives detonate. With investors ready to move ahead, a reluctant Mackintosh has his henchman kill Lightfoot, but Frost quickly deduces his involvement, thanks to Burrell, and arrests Mackintosh and his henchman.

***** S10E03 Held in Trust
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Frost finds his belief in the law tested when his latest case involves known paedophiles and missing and abused children. A child, Bobby Palmer, goes missing while with his father at a football match, and a murdered child, who was beaten and malnourished, is found in a landfill site. The malnourished boy is traced to a family who bring runaway kids into their home to portray as their own children to claim additional benefit funding. When the boy developed tuberculosis and seemed unresponsive, the father assumed he was dead and dumped him at the landfill site, where he eventually died.

Frost's investigation of an apparent hit-and-run reveals the man, Peter Johnson, was bludgeoned beforehand. He learns that Johnson was a paedophile, seemingly keeping his urges under control, but was spotted associating with unapologetic paedophile Anton Caldwell. Frost follows clues in Caldwell's taunts during an interrogation, which leads him to an abandoned railway line where he uncovers Bobby's body. Meanwhile, Frost's colleagues encourage him to go before the promotions board, while behaving himself, to apply for the role of Chief Inspector. Although Frost leaves the interview midway to pursue his investigation, he receives the promotion.

Forensics reveal that Caldwell was involved in Bobby's murder. The investigation uncovers that the Palmers were visited by a social worker who accused Bobby's father of abusing him. Frost learns no such visit was recorded by social services and that Martin Drew was fired as a social worker after becoming obsessed with understanding paedophiles, bringing him into contact with Johnson and Caldwell. Frost finds home videos of school children at Drew's home and learns another young boy has been kidnapped by Caldwell. Frost and his team quickly catch Caldwell and Drew, who reveals that they killed Johnson because he tried to stop them. Frost loses his temper when Caldwell states what he did to his victims, and punches him. Caldwell presses charges against Frost, leading to an enquiry, and though he has the backing of his team and Mullett, who will swear Caldwell was resisting arrest, Frost refuses to lie in court. Frost is placed on indefinite suspension, costing him his promotion, and he accepts that this will likely be the end of his career. Frost's colleagues applaud him as he leaves.