The final part of the trilogy is the shortest, and it probably doesn't make much sense unless you have read at least Tinker, Tailor, Soldier, Spy. I very vaguely remember the original broadcasting of the BBC adaptation (and I didn't have a clue what was going on); I've since watched the DVD, and I still think it's one of those things you have to know already in order to understand it.
Power corrupts, and absolute power corrupts absolutely. It transpires that Karla has been embezzling funds in order to pay for care and medical treatment for his (illegitimate?) daughter in Switzerland. This comes to light through the hounding of the widow of a Russian emigre in Paris, in order to provide the girl with a suitable false identity for settlement in Switzerland. The news reaches London, but news of the news itself is also on the move, and the "General", elderly leader of the group, is murdered on Hampstead Heath before meeting with his Circus contact. At this point Smiley (the group's "vicar") is summoned out of his (recurrent) retirement to attend to the case, unofficially, as it is deemed too sensitive for the current government. Smiley has to retrace the network's steps in order to discover the message that was not relayed, and this gives him the excuse to revisit various old contacts such as Toby Esterhase and Connie Sachs, with others (including Karla) collectively providing the book's title. Once the evidence is collated, the trap is sprung.
"George, you won," said Guillam as the walked slowly towards the car.
"Did I?" said Smiley. "Yes. Yes, well I suppose I did."
It's a slow burner but rewarding for those prepared to stay the course.