If you want a Mission Impossible style bang fest or a Bond style romp this isn't your thing. If you need a plot recap after every ad break you lack the concentration for this.
But if you want a drama that engages, immerses, and makes you think then this will do the job.
Some reviewers have said it's slow, drab, tedious, the constant changing between timelines is distracting and that Mrs Thomas is an anachronism.
I think they're missing the point.
This is only 20 years after WWII, 20 years after these events government offices looked little different. Anyone who was in the Civil Service during that period will know the details are accurate.
The tension and mistrust between CIA, the Security Service, and SIS that followed the war, Blake, Burgess, and McClean is abundantly clear and so to the extent this hampered both their efforts.
One must also remember that this was only 2 years after the Cuban Missile crisis and the suspicions of East about West and the efforts and paranoia of intelligence and counter intelligence operations is very apparent.
This drama is a mind game that reflects the way that Philby manipulated his friends and acquaintances over decades. The constant reviewing of events by those involved akin to the thoughts of a cuckolded husband as he tries to spot what he has missed. The sense of betrayal almost palpable.
The drabness, the politeness, the ordinariness in stark contrast to the fatal impact of Philby on those he betrayed.
Pearce is exceptional as the idealistic Philby and there is an almost physical change as the realisation of his decisions and the realism of living in the Soviet Bloc hit home.
If he had been born half a century earlier Lewis could have been Elliot in reality. I couldn't make up my mind if he was an exceptional actor, or not acting at all he was that convincing.
And as for Maxwell-Martin, yes, Mrs Thomas was made up. My interpretation was her character aptly contrasted with the old boys club of Philby and Elliot and was created for this purpose. If SIS and the Security Service had had more like Mrs Thomas to challenge the status quo then perhaps the Soviet moles might not have burrowed so far into the establishment.
So stick with it. I thought it well worth a few hours of my time.