The real story, the story underneath the mechanics of who the suspects are, how to locate them, what has been compromised, is that political ideology has no reality. Humans work to uphold political ideals, but those ideals don't make anyone's life any better. It's the dispiriting realization that all the members of the Circus work to keep Britain safe from the Soviets, but that their lives as lived are no better for it.
The emotional cosh comes in some off-hand moments, especially at the end. [SPOILERS!!]
Gary Oldman, as George Smiley, goes to "The Cottage," a small cabin in a wood, fenced in and topped with barbed wire, where the captured mole is being held until he can be returned to the Russians in some unexplained diplomatic exchange. The mole here is Colin Firth, standing a bit awkwardly in this rustic location--he was "Tailor" and his urbanity is at odds with the natural surroundings. This is a man for cities, not trees and falling leaves. As he smokes casually, he unburdens himself to Smiley. I don't even remember if Smiley asks him any questions, even, but it is obvious what he wants to ask--Why?
"It was as much an aesthetic choice as anything," Firth says, blowing out cigarette smoke. "Britain had become so ugly." And that was the revelation, at least for me.
My experience of the Cold War was most vivid during the Reagan years, when those Bad Soviets drove so much of Reagan's world view and foreign policy. And at that time, we were deep into the 1980s aesthetic, where big hair and asymmetric clothes and neon colors signaled an era of excess. Greed was good, fortunes were being made in mergers and acquisitions, luxury goods were trickling down into department stores and within the reach of more and more people.
Oh, sure, it looks silly now, but at the time it was such a breath of fresh air after the drab Earth-shoe earnestness and ugliness of the 1970s. The Soviet Union, in contrast, remained stuck in the aesthetic of concrete construction and bland clothing. The popular image was of ugly people in drab clothing waiting in long lines to just buy bread.
Well, isn't it obvious how much better Western freedoms are? Sure it is! That's why the Soviet Union collapsed--Mikhail Gorbachev let his lovely wife Raisa go shopping in America, and it was all over!
What is apparent in every detail of the production design of Tinker, Tailor, Soldier, Spy is that life in Britain of the 1970s wasn't much different than life in the Soviet Bloc. The work done by the Circus was no more noble or enriching than the machinations of their opposites of the USSR. That Colin Firth could stand on English soil and believe that Russian life was more aesthetically pleasing felt accurate after the two hours of living in the recreated world of the movie. Britain was ugly, and was about to get uglier as Thatcher policies inflamed class resentments. The thing about this mole--he was a traitor to the country, but he wasn't wrong.
The movie ends with brief vignettes of the lives of the characters we have been seeing, including Smiley walking into his home and approaching his faithless wife. She betrayed him with Bill Haydon (Colin Firth), herself a victim of Haydon's deliberate campaign to seduce her. He was under orders to do so, because the Soviet Spy (Karla) believed that this would exploit Smiley's weakness and make it harder for Smiley to see Haydon accurately and allow him to continue operating as a mole. And it worked--to a point.
So the future relationship between Smiley and his wife is now compromised--she was unfaithful, but she was targeted because of Smiley. How much is he to blame? And equally important, what else does he have to keep him tethered to life without her? She is far from perfect--is she better than nothing? What other options does he have, given who he is and what he does?
So much of this movie is told through the production design--the changing world signaled by the passing of the Old Guard and their short hair; the rise of the New Generation with their longer hair and casual shoes; the spare ugliness of the buildings and materials that surround the Circus; the ugly cars and grubby streets. Even the use of color is so restrained--some of the only bright spots are Benedict Cumberbatch's matching tie and handkerchief, which are themselves coded to convey his character's homosexuality.
Finally, the reality of the British spy game is so demoralizing because the whole point of Karla's putting a mole into The Circus was not because he had any interest in British intelligence--Britain was virtually useless and marginalized, a tiny island of no value. Karla was trying to hook the Americans, and using The Circus as bait for the big fish. All this human cost, all this pain, all these lives sacrificed and destroyed, and they were only the pawns of a larger game.
Smiley is no James Bond, and spy work is not glamorous, rewarding, or even useful, and the more clearly you see the work of espionage, the less you can see the difference between one side and the other. That's the real story LeCarre was telling, and that's the real story of the movie.