Not being familiar with the source material upon which this is based, I worried that i would have a hard time following the various plot entanglements upon learning it was based on a rather knotty British spy novel but the screenplay makes everything that is happening (and why) more or less clear enough for someone just catching it randomly (like i did) to get.
Film's first half is really quite good. It takes you into the cold and somewhat distant world of this quite average bureaucrat's office and then home life (wherein you learn that he's being suspected of leaking top secret info--"very unimportant top secret info" as one of the characters making these allegations says--but top secret none the less.) Film really captures and sustains a very solid tone of slow growing mistrust by everyone in the film. Nicol Williamson's character realizes he's under suspicion right away but the various other characters' slow burning distrust of one another grows rather nicely as the film goes on. (In true British spy film tradition tho none of them ever seem to voice their distrust of one another to each other, choosing instead calmly and carefully constructed parables of being trapped in boxes within boxes.) As Nicol Williamson somewhat slowly tries to put together a plan to quiet his superior's suspicions about him and keep his wife and adopted son safe, everything around him naturally falls apart and the film's narrative somewhat suffers from having to keep clear the reasons why Williamson is doing what he's doing as well as what his superior's are up to (and why they too are doing what they're doing) It doesn't help that the film without much warning about halfway through flashes back to Williamson's time in Africa when he first met and began an affair with the woman who would come to be his wife---that part is of course supposed to explain Williamson's motivation and give you some idea of what's at stake--but because of its rather abrupt happening, you never really feel much except for confusion, especially once the film just as arbitrarily jumps back to present day.
Film's last half hour or so gets somewhat jumbled and almost completely loses the nice quiet momentum it had been steadily building up---jumbled as in i'm not entirely sure what exactly happened but I get the main idea regardless. I won't spoil the events that happen but I will say that the big event that happens at film's end certainly doesn't feel like a big event, nor does it feel like a very satisfactory ending. Perhaps the novel was able to end this way because it was able to leave you with the proper sense that the main character's feelings on what is happening to him was resolved more or less, but the film literally leaves you hanging for much, much more closure on behalf of the fate of one Maurice Castle.
That said there are some wonderful details featured throughout. From Robert Morley's delicious performance as the devious doctor whom the section employs for "physicals" of its employees, to the wedding reception at the easily cowed secion chief Richard Attenborough's house (wherein his shrew of an ex wife completely chastises him for breaking her precious ceramic owls) to the painting of boxes decorating the hotel walls where the villains meet in the first half to discuss what is to be done with their suspects to just about every scene featuring Derek Jacobi as the heavy drinking/no sweat colleague of Castle, there is much to be savored and enjoyed here...just don't expect any action or actual resolutions to the film's various plot points.
