Conversion

The perfect crime, as planned: a team of bank robbers gets a gullible youngster to hand over the local knowledge, then turns up on the street dressed as gas workers, with the right names and the right story, and walks into the middle of the neighbourhood without anyone raising an eyebrow. The disguise is the masterstroke. Nobody questions a man in a council-issue overall carrying a clipboard.

The police are left chasing men who were never really there. Watt's squad has to unpick the deception from the outside, and the episode's tension comes from watching the robbers stay calm while the net slowly tightens. Billy Hamon's Pete Ryan, Frank Barrie's Spike Harran and Graham Weston's Tom Bishop are a plausible crew, professional enough to run the scheme and careless enough, as these men always are, to leave the one thread that will unravel it. Mary Quinn's Mrs. Wilson is the civilian link, the person the plan ran through without her ever knowing it. The scheme works because the people it is run on are busy and trusting, and a uniform and a clipboard beat suspicion almost every time.

Elwyn Jones wrote it, Geraint Morris directed. The regulars are out in force: Watt, Cullen, Adler, Evans, Snow and PC Nesbitt, played by Grahame Mallard, who is becoming a familiar sight in these episodes. The squad's patience is the show's great asset, and it is on full display here. It has to be, with a plan this neat.

"Conversion" is the kind of episode the series does with its eyes closed, a competent, mid-paced procedural about a scheme that looks clever until you shine a light on it. The gas workers disguise still lands, though. Fifty years on, it remains one of the more quietly audacious plans in the show's history, and it is the reason the episode holds up.