If Dogs Run Free (1)

The final two-parter opens with Frost assisting RSPCA inspector Christine Moorhead in dismantling an illegal dog-fighting ring — and with the start of the last romance of his life. The raid is a success that fails: the ring's organiser, Gerry Berland, a wealthy businessman with his fingers in drug money and laundering, arrives after the police have gone, tipped off by a delay. Berland is untouchable, and he knows it.

The first death follows the raid. Berland's thuggish son Sean, convinced his ex-friend Brian tipped off the RSPCA, attacks him in a scuffle and stabs him dead. Christine recognises Sean — and his unwitting accomplice Neil — as pupils at the same school as her own children. Berland, smooth and merciless, arranges alibis for the boys and coerces his wife Sally and Neil's parents into silence, though both boys are drowning in guilt and want to confess. Christine is the only witness, and Berland knows it. He intimidates her; when that fails, he sends his drug-addicted thug Lisowski to burn the RSPCA unit down with Christine inside.

Alongside the dog-fighting case runs a thread of dread: someone is replaying crimes from the early days of Frost's career. A mugging, a murder, Frost's house vandalised, and the words "You Die Next" scrawled on his car. The series, which has always been about the past coming back, is setting up its final confrontation.

If Dogs Run Free (1), written by Michael Russell and directed by Paul Harrison, is a season finale that plays like a beginning — a new romance kindled in the ashes of an arson attack, a villain who has finally met his match in a witness who will not be silenced, and a threat to Frost himself that the second part must answer. Adrian Dunbar's Gerry Berland, all charm and menace, is a worthy last opponent for the scruffiest detective in British television.

A finale that plays like a beginning — a new romance, a formidable new villain, and a threat scrawled on Frost's car. Adrian Dunbar's Berland is a worthy last opponent. A finale that plays like a beginning.