Ep by ep; To Catch a Rat
My memories of this one are clouded as I remember reading the novel as a kid and being horrified, the violence of it was incredible, the lead villain murdering an agent by throwing him under a train with his severed head bouncing onto the railway siding, murdering another by throwing her down the stairs, killing a priest at the graveyard with a shovel and murdering the hero's girlfriend by chloroforming and raping her. When I watched the finished ep there was none of this and I wondered why the hell the writer had bothered to put it into the adaptation? Even the Professionals and Sweeney would have baulked at it.
All told this is a pretty gold Cold War story and a lot more hard nosed than usual, The Professionals would reuse part of the storyline in several eps, you couldn't have this every week but it makes a nice change some times from some of the sillier eps.