You have to be alert for this film.  Starring Gary Oldman, John Hurt, Benedict Cumberbatch, Colin Firth, Tom Hardy, and other recognizable faces, Tinker Tailor Soldier Spy is story about finding a Russian mole in the top echelon of British intelligence (aka "The Circus").

The movie is based on a book of the same name by John le Carre, the first of the "Karla" trilogy (but complete on its own).  In it we follow George Smiley (as played by Gary Oldman) as he investigates Control's (John Hurt) theory that they have a Russian mole in the Circus.  In an irony that would make my high school English teacher proud, I'm pretty sure Smiley never actually smiles.

Unfortunately the tale is told through flashbacks, revealing tidbits of information when relevant, but you have to stay on the ball to know "when" each segment occurs.  The order of events, once you're done the movie, will greatly help your understanding of the movie.  I found this page helpful (but only visit once you've seen the film): http://malnurturedsnay.net/2011/12/tinker-tailor-soldier-spy-i-can-explain-it-to-you/

Although there's a lot of death, it's a VERY low action movie.  Most of it is spent watching Smiley walk from place to place, or move from chair to chair, or look out windows and remember some fact.  These are the people who pull the strings.  It's not a Bourne movie.

I'm amazed this movie got an R-rating.  There was only one on-camera death, which was pretty mild compared to prime time TV, and roughly five seconds of boobies in a background scene.  There wasn't even any swearing!  Just smoking, walking, talking, and sitting.  A raised voice here or there.  Do they just think younger viewers couldn't sit through a thoughtful film??

The Good: Stellar cast.  Interesting how minute details link up important connections.  Audience not led by their nose.  Cinematography "feels" like 70's spy series (and I don't mean in a campy way)
The Bad: Not sure flashbacks was the best way to tell the story
The Ugly: Maybe I was too tired, but it was confusing enough to send me looking for an explanation online

There was also a 1979 mini-series starring Sir Alec Guinness as George Smiley (and Patrick Stewart as the Russian agent "Karla").  Might be worth a watch!