Only the very mildest of spoilers here!
Watching the series chronologically, this is probably the best episode so far, and I'd be very surprised if it isn't one of the series' very best.
A frail, elderly English woman living in Brussels is approached by a man who identifies himself as an English airman. The English woman's son had been best friends with none other than Major Brandt, one of the leading hunter's of escapees. The story becomes more complex as a secret plan by Gestapo officer Kessler is revealed, the escape train are faced with making rapid and controversial decisions (as are the other characters), the Brussels populace are under pressure to celebrate Hitler's birthday, and Mdm Foiret's suspicions about her husband's other activities are fed. Almost ever character is both manipulating and manipulated, motives are complex, and no one behaves quite as might be expected, yet remain true to character.
The series, while always strong, can at times fall into the trap, in endeavouring to be realistic, of simply presenting relentlessly downbeat endings, but this episode rather avoids that. It is truly gripping, with resolutions which, if not entirely conclusive, are satisfying and illustrate depths to our familiar characters which have only been glimpsed before. The episode also stands pretty well on it's own, for anyone wanting to sample a strong, if perhaps not quite typical, episode. It does give a flavour of the series.
The only criticism I can think of is that a Fuhrer's birthday parade is unfortunately staged in the studio on video tape, with what looks like at most 20 extras rather than via location filming. But that is a small criticism in an episode where one is constantly thrilled (in 1970's TV drama terms), surprised and entertained.
A small masterpiece of complexity and suspense.