Thin Air (1)

Twelve years is a long time to keep a dress. Thin Air (1) opens with exactly that turning up: the dress Joanna Gold was wearing when she vanished from Hampstead Heath in 1989, found in a lock-up, and suddenly a disappearance the whole of London remembers becomes a murder hunt again.

The case has always had a terrible fame to it. An eighteen-year-old girl, gone in broad daylight, in a park full of people. The team starts pulling at the threads around the Gold family, and the episode is good at showing how much a family can be bent out of shape by one afternoon. Roger Allam and Cherie Lunghi play Benjamin and Leah Gold, the parents, with Sophie Winkleman as both Joanna and her sister Clara, and George Calil as David Gold. Jack Ellis turns up as the Police Commissioner, which tells you how high the interest in this one reaches.

Boyd, meanwhile, is having a bad time of it. The pressure of the case gets to him, and he makes mistakes, several of them, the sort that matter in an investigation this watched. It's one of the better decisions the show makes with him: he's brilliant at reading people and hopeless at being read, and here his own blind spots start costing him. Barbara Machin and Ed Whitmore wrote it, Edward Bennett directed.

The regular squad is all present: Sue Johnston's Dr Grace Foley, Holly Aird's Dr Frankie Wharton, Claire Goose's DC Amelia Silver, Wil Johnson's DS Spencer Jordan.

What stays with you is the Heath itself. There's something properly unnerving about a place so ordinary holding a secret this old, and the episode lets that unease build slowly rather than going for shocks. Grief sits over the Gold family like weather. And the hour ends with the killer still out there, which is exactly where a first half wants to leave you: annoyed, hooked, and already reaching for the remote.