Some Must Watch… (master); IMDb styles it "Some Must Watch"

Special Branch are watching a Territorial Army centre when they spot what looks like a raid on the weapons store. Armed officers respond, and a 68-year-old workman is shot dead by mistake. That's the kind of opening that should produce a clean, awful investigation. It doesn't. But the supposed intruders claim to be working for MI5 and refuse to help Clark at all, and the official line turns out to be a whitewash.

The truth, when it surfaces, is that MI5 had been bugging the centre to entrap a Special Branch Inspector and an N.C.O. suspected of selling arms to terrorists. Clark's investigation is collateral damage in someone else's operation, and the security services' reach into his work is the real story. Rob Heyland wrote it, Roy Battersby directed, and Francesca Annis is quietly excellent as Angela Berridge, the Home Office official who becomes his reluctant lifeline. Oliver Ford Davies, later of the Star Wars prequels, plays John Gollap, and Michael Byrne's MI5 Director is the unblinking face of the machine. The cast list runs deeper than the headliners: Chris Gascoyne, more than a decade from Coronation Street's Peter Barlow, is Nottle, and Jaye Griffiths's Molly Cope adds another face to the enquiry.

The regulars recede a little, which is right; this is an episode about the scale of the opposition, not the usual suspects. Tom Georgeson's Naylor and Siobhan Redmond's Connell keep the station end moving while Clark wrestles with an organisation that doesn't answer to him and doesn't have to.

It's the season's spine showing through: a copper who thinks he's investigating corruption, discovering the corruption is investigating him. The mistake at the heart of it, the shooting of the workman, gets buried under official priorities, and that's the episode's quiet horror. Nobody sets out to kill an old man carrying out repairs; the system just has other things it wants to protect. The whitewash isn't a scandal to the people doing it, it's housekeeping.