***** S15E01 If Dogs Run Free (Part One)
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Frost and his team assist RSPCA officer Christine Moorhead in dismantling an illegal dog-fighting ring, but its organiser, drug lord and money launderer Gerry Berland, avoids arrest after a delay makes him arrive after the police raid. Believing that his ex-friend and fellow teenager, Brian, tipped off the RSPCA, Berland's thuggish son, Sean, attacks Brian, and in the scuffle stabs him to death. Christine recognises Sean, and his unwitting accomplice, Neil, as they attend the same school as her own children. Berland arranges alibis for the boys, though both are overcome with guilt and want to cooperate. He coerces his wife, Sally, and Neil's parents to cooperate to keep the boys out of prison. Aware Christine is the only witness, Berland intimidates her and, when that does not work, has his drug-addicted thug, Lisowski, set the RSPCA unit on fire with Christine inside.

Meanwhile a copycat criminal is replicating crimes from early in Frost's career, leading to a mugging, a murder, Frost's house being vandalised, and "You Die Next" being scrawled on his car.

***** S15E02 If Dogs Run Free (Part Two)
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Christine survives the fire relatively unscathed, and she and Frost bond romantically during their time together. Toolan's investigation of the copycat reveals a description fitting the station's new archivist, Jennifer. At her home, he and Frost find obsessive news clippings of Frost's cases and realise Jennifer is the daughter of his old colleague, whom Frost exposed for corruption, and spent the last few years of his life trying to find something to bring Frost down as well. Frost was close to Jennifer as a child, but her father's obsession has passed to her. Jennifer lures Frost to a rooftop where she orders him, at gunpoint, to jump, but Frost tackles her and she is arrested.

Upset that Lisowski's arson attack was too excessive, Berland gives him a pure dose of cocaine which causes him to die of an overdose. Frost suspects Berland is involved but is unable to link the two men until Berland's solicitor, Greg Salmond's car is stolen, used in a hit-and-run of a young girl, and found abandoned with a large stash of cocaine in the rear. His investigation leads Frost to arrest Salmond's colleagues, who used the drugs and took the car on a joyride, while CCTV shows Salmond purchasing the drugs from Lisowski. Frost leverages Salmond for the location of Berland's main drug, money, and dog fighting operations at a farm, leading to the collapse of his empire. Overcome with guilt and with no one to talk to, Sean hangs himself, and his distraught and long-abused mother leaves Berland. Devastated over his loss, and now realising what Brian's parents must have felt, Berland surrenders himself to Frost. Neil and his parents decide they must tell the police what they know, as it is what Sean wanted.

Frost asks Christine to marry him. Her seemingly affable but actually relapsed alcoholic ex-husband, Allen, meets with Christine after the wedding rehearsal to plead with her to get back with him, but she rejects him. On the wedding day, Frost, and his best man, Toolan, arrive at the church and are greeted by Mullett, before all three are deliberately struck by Allen's car. Allen is killed instantly, and Toolan later dies from his injuries, with his wife, Frost and Mullett at his bedside.

Some time later, Frost bids farewell to George's coffin. He tells Christine that he has long dedicated himself to his job, which he considered to be a refuge, and gave up on having a personal life, unlike Toolan, but he realises now that he must prioritise his personal life, together with Christine, noting that he has a lot of time to make up for.

***** S15E02 If Dogs Run Free (Part Two)
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Alternative endings were written and filmed, depicting Frost dying instead of Toolan.