Appropriate Adults

Eight-year-old Tricia Martin disappears, and the final person to see her was Billy Conrad, a kind-natured nineteen-year-old with Down's syndrome — a young man the local children adore and the town is suddenly, terribly willing to blame. Frost is working the case under a new DCI, Hawkes, a promotion-hungry officer from outside who wants a quick confession, and the episode is built on the collision between Hawkes' ambition and Frost's insistence on the truth.

Billy will not explain where he was when Tricia vanished — the truth, which he conceals for fear of being separated from her, is that he had been visiting his girlfriend Jane, also a young woman with Down's syndrome, at her care home. When Tricia's body turns up in the woods, Hawkes interrogates Billy into confessing to the murder. Frost, unconvinced, digs into Tricia's own history and finds the cracks: she had been taken to hospital with serious injuries put down to a fall. Her father Richard, it emerges, was physically abusive, and joyriders Frost is investigating saw a man matching Richard's description near the woods. Cornered, Richard confesses: he struck Tricia in anger, she fell and hit her head while fleeing, and he hid her body, unable to face telling his wife what he had done. Billy is released — but his reward is to watch Jane being moved away from the care home, to keep the two apart.

The second thread is a security farce with teeth. Frost uncovers that Parker, a man claiming to be an officer supposedly on leave from another station, turns out to be an impostor who embedded himself with Hawkes' pool team and used the access to wander the station at will. It is a neat jab at the very systems Hawkes represents.

Appropriate Adults, written by Michael Russell and directed by Herbert Wise, is the series at its most morally urgent. Matthew Marsh's Hawkes is a magnificent villain of procedure — not evil, just lazy and ambitious, happy to imprison a young man because it is easier than investigating. Timmy Lang's Billy, guileless and wounded, gives the episode its heart. The title refers to the legal guardian who should have been in the room when Billy was questioned; the episode's quiet horror is that no one insisted on one.

The title is the whole argument: a legal safeguard that should have protected Billy, and a system too lazy to provide it. It remains one of the most discussed Frost episodes for good reason. It still holds up as one of the show's finest hours.