Held in Trust

Bobby Palmer disappears while at a Denton Town match with his father, and the case lands on Frost with the weight of everything the series has ever said about the vulnerable. The investigation is already dark when a murdered child — beaten, malnourished is discovered in a landfill; the boy is traced to a family that that took in runaway children, presenting them as their own to claim extra benefit money; when the boy grew sick with tuberculosis and seemed unresponsive, the man of the house assumed he was dead, dumped him in the landfill, where he died.

The Palmer case moves through a fog of monsters. A hit-and-run victim, Peter Johnson, turns out to have been bludgeoned first — and Johnson was a paedophile, holding his urges in check but spotted with Anton Caldwell, an unapologetic predator who taunts Frost during interrogation. Following Caldwell's clues to an abandoned railway line, Frost finds Bobby's body. The trail leads to Martin Drew, a social worker fired for his obsessive interest in paedophiles, and to home videos of schoolchildren at Drew's home — and to a second boy, already kidnapped, whom Frost's team rescues just in time. Johnson, it emerges, was killed by Drew and Caldwell for trying to stop them.

The episode's moral weight lands on Frost himself. When Caldwell describes the things he did to his victims, Frost loses his temper and hits him; Caldwell presses charges; an enquiry follows. Mullett and the team are ready to swear Caldwell was resisting arrest — but Frost will not lie in court, and accepts indefinite suspension, costing him the Chief Inspector promotion he had finally, reluctantly, pursued. His colleagues applaud as he walks out.

Held in Trust, written by David Gilman and directed by Roger Bamford, is the series' bleakest hour — a case about the people society fails, pursued by a detective who destroys himself refusing to join the lie. Nicholas Woodeson's Caldwell, serene and monstrous, is the show's purest villain, and Frost's punch is the moment the series finally stops pretending its hero can stay clean.

The series' bleakest hour, in which Frost punches a monster and destroys his own career rather than lie about it. The applause of his colleagues as he leaves says everything. The bleakest hour, and the one that defines Frost.