I have to be careful about this one; it's clearly a quality piece, and has gained rave reviews, but personally, I haven't much positive to say about it sadly.
The story is quite straightforward in many ways. There has been a little coup at MI6, and Control (John Hurt) and his right hand man George Smiley (Gary Oldman) have been forced out, in the wake of some of his subordinates having done an end run around him and set up an immensely valuable intelligence source, despite his reservations and suspicions.
However, it comes to light later on, after Control's death, that one of the top men at the Circus (MI6 HQ), is a mole, one of the very men, perhaps, responsible for running this new source. Hence, the Ministry bring Smiley out of retirement, and set him to smoking the mole out.
The film plays out as a sequence of short, often silent vignettes of Smiley entering a room whose significance isn't immediately obvious, sighing, looking pensive, and then cut to the next bit, where someone else does likewise. Information about what's going on dribbles out at a miserly rate, until we finally have enough to piece together what's going on. And this is what defied my best efforts to engage with the film. It's not just that there's scenes where nothing much happens. It's that there's lots of very short scenes in a row, apparently nonsequiturs, in which nothing much happens. Maybe it's because I was tired, but I just couldn't get into the flow of the film, and I think it was entirely down to the directorial and editing style.
Which is not to say I didn't get anything out of it; the whole British acting community turns up, in order to prove that there's life after Harry Potter, and it's like gladitorial combat of powerful but quietly understated performances. And the betrayals are so numerous, and the reasons so obscure, that there's much to be gained from thinking about what happened after the event. Actually watching the film, however, felt like a chore at times.