'SS-GB' takes as its source material one of Len Deighton's novels, certainly not his best but likely his, er, splashiest, conceptually (what if the Germans won the Battle of Britain?) and remains ~mostly~ faithful to the book for the first five episodes before making a series of plot adjustments, some for the better, others not so much.
I'm going to try and talk about this without spoilers, but I will say, at the outset, that one thing that having read the book *prior* to watching helps with is that the miniseries does not do a particularly good job explaining the internecine conflicts within this fictional world's German police, military, paramilitary, and political classes. I make no claims as to the *book's* accuracy in terms of its real world counterparts, but its internal explanations are coherent in its 'alternate history' world--the miniseries does not have the patience (or perhaps screen time) to lay this all out. The problem with this is that a significant number of plot points are strung on a framework of knowing this so that, I suspect, the miniseries is rather more difficult to follow. And, as a result, other, less interesting aspects of the story had to, either, be brought to the fore or are just forced to support more of the plot by virtue of their presence. I'm not trying to suggest that you *need* to read the book first, but be aware that there are issues.
The elevator synopsis is that Sam Reilly, playing Archer, an English detective in London & reporting up a German command chain, is called out to investigate a murder which, initially, seems to be related to the black market that is thriving under the Nazi occupation of England (or are least the lower half of it). Naturally this spirals into something much more convoluted, involving conflicts between the German army and the SS (and within the SS itself), members of the resistance and those who are just trying to get by under the new regime, and between Archer and himself over divided (and perceived divided) loyalties, while the effects of the murder become ever more international in scale.
The series is a BBC production. In the past, I'm going to call it the pre-high def era, seeing 'BBC' on the credits came with a set of, not exactly 'expectations,' but more an awareness of, let's say, limitations to do with budget (these shows were not being produced in the American fashion of 'prestige' shows--regardless of whether you like them or not, I'm talking about stuff like 'Mad Men,' 'Deadwood,' 'The Sopranos,' and so on), as well as effects, casting, and so on.
Now that we have high definition for tv shows though, this creates a problem for lower budget projects like this one because, whereas in the past, flaws could be more easily hidden, that's no longer the case. As a result you either have to just embrace it and look 'cheap' (which, I must add, *can* work fine) or you have to go smaller scale. 'SS-GB' has opted for the latter which means that everything feels really tight. Exteriors are shot at night or, more commonly, on short streets and crowd scenes are shot close, highlighting only a few people at a time--this makes everything look claustrophobic which is the exact opposite of the way the story (with each revelation unfolding to encompass something wider) progresses. Had this been an original story rather than an adaptation, those differing tendencies could have been made to work together, but here, the limiting production just kills the story's scope.
The performances are ok, though as I've complained about in other films/tv shows, none of these actors look like someone from the 40s (even an 'alternative' 40s). Reilly's Archer can wear a trench coat (though he'd nailed that as Ian Curtis in 'Control') and hat like no-one's business, and has adopted an interesting, flat mode of speech which, far from being bad (as I gather some have found it), I think accurately reflects Deighton's character's mood. The actors playing Germans are bit more of a mixed bag, with accents slipping and sliding every which way (something you don't see terribly often when Brits play Germans) and some pretty hammy acting, the latter of which is also true of more than a few of Brits playing Brits (thank god there weren't *more* Cockney characters, guv'nor, what?). As for Kate Bosworth, she's alright (the make-up and costuming covering some anachronisms) but, as written by Deighton, doesn't have a lot to do beyond intrepid, American gal-reporter stereotypes.
As I mentioned at the beginning, the series excises one subplot in particular, which greatly affects the relationships of two characters, even down to whether one survives to the end or not. As it stands, its removal results in a slightly tighter last third, but robs the actual conclusion of a fair bit of its pathos. It also, inexplicably, adds a whole additional scene to inject some 'action' where none was needed, and blithely retains Deighton's worst plot convenience (regarding car engines). On the plus side, the series fixes a few things, like removing an awkward, tentative romance feint early on, finds a clever solution to blocking during a bombing sequence (though it also fudges this by reusing shots), and, by virtue of being a visual medium, is able to better demonstrate some of Archer's interactions with various coworkers in the police station and how that feeds into his own inner turmoil.
That all said, I didn't enjoy watching this all that much and was kind of just waiting for it to be over. A middling adaptation of a middling book, albeit with a conceit that should have elevated both beyond what we've been given.