It's a startling break of pace with the rest of the series. It's set in 1969, so almost 25 years after the final episode that was shown but still ten years before it was made. We have the old gang coming back together to identify Kessler's new identity as industrialist Manfred Dorf, as part of a British TV show about the resistance. It just doesn't work dramatically. Secret Army was about a time and a pace, and the experiment of unmooring the show from its context can sometimes work (eg the last episode of The Prisoner), but here it doesn't.
The final scene doesn't work very well at all, with Natalie expressing her dedication to fighting Communism everywhere; although Communists were also bad guys on the show, this conclusion doesn't really seem to follow from the premises that we have. And the floaty effect is really awful.
In parenthesis, writers of big works like this often do like to do a twenty-years-on epilogue. Tolkien wrote one for The Lord of the Rings, but was persuaded to drop it. J.K. Rowling wrote one for Harry Potter, and should have taken the advice to drop it that I'm sure she was given. Often it's better to let the story stand for itself, without the writer popping in to tell us what it all means.
So, on to Kessler, shown in November and December 1981. I didn't watch it at the time; at 14 I think my parents would have allowed me to if I did, but I wasn't interested. It is set around the present day (hints that it may be late 70s rather than early 80s), and starts with the reunion in Brussels from the unshown What Did You Do In the War, Daddy?, but then develops into a story of West German anti-Nazi security agent Richard Bauer and freelance Israeli Mical Rak, played by Alan Dobie and Nitza Shaul, pursuing Kessler to try and expose his new identity and his plans. They chase him to Paraguay (these scenes filmed in Spain) with his daughter, herself a convinced next-generation Nazi. After some running around, Kessler's daughter is killed, he takes his own life and justice is served. I must say I found it minor stuff.
It does vaguely capture the fragility of the Bonn Republic, which always felt a bit more provisional than it professed to be. But it's disappointing not to get the context for Bauer's work; we sense that he is also freelancing in his pursuit of Dorf/Kessler, but it would have been fair to West Germany to unpack that a bit more.
There is one really outstanding episode, the fourth, in which Kessler arrives in Paraguay to face interrogation by the senior Nazis there, headed by Josef Mengele, played memorably by Oscar Quitak. In reality Mengele died in 1979 in Brazil, but this was not known until 1985, so at the time the show was made he was thought to be the senior escaped Nazi. (Martin Bormann also makes an appearance, though the consensus since the mid-1970s is that he died in 1945 and that his remains were discovered in Berlin in 1972.) The scene where Mengele and his colleagues discover that Kessler has been tailed to Asuncion by Rak and Bauer is pretty good.
There is a book as well, Kessler by John Brason, and the second paragraph of the third chapter is a solid bit of exposition:
Half-an-hour later Rückert stopped the machine playing, and pressed the re-wind as he turned to Kessler. Both men were silent and unhappy. They had watched snippets of interviews with Nazis living in comparative security in various places in South America. Pathetic, frightened creatures who lived in the bars and shadows of towns and cities from Tierra del Fuego to Buenos Aires and Maracaibo - old faces of old comrades-in-arms. Bur they thought most of all of an interview between van Eyck and the man calling himself Manfred Dorf, head of the Dorf industrial empire, a respected international industrialist being accused of false identity ... the truth being that he, Dorf, was none other than Standartenführer Ludwig Kessler, one time Head of SS and Gestapo in the Low Countries, the much-feared Höherer SS und Polizeifiihrer from 1942 to 1944, a territorial leader with the corresponding significance of Gauleiter. The lack of birth papers, the absence from census, the startling likeness to the military records photographs, the coincidences, and possibly the supreme coincidence of the Belgian wife whose name just-happened to be Madeleine.Unlike the Secret Army books, this is a straightforward novelisation of the complete TV series. (The first Secret Army book is a prequel, and the other two mix novelised TV episodes with original material.) In the days when home taping was something you did by holding a microphone up to the TV, this was the best you could get to relive the show, and it's a faithful account; the good bits are still good, and the background exposition is still not as full as I would have liked. You can get it here. (the DVD of Kessler is no longer available, but of course it can be obtained in the usual way.)