George Smiley had been a senior figure in British Intelligence but he was forced to retire along with his boss when an agent is killed on an unauthorised assignment in Hungary. However rumours of a Soviet mole at the top echelons of Intelligence (known as the circus) have persisted and the Minister in charge decides to find out whether they are true once and for all. Smiley is chosen to do this as he knows the spy business and all the suspects very well but is no longer working with them.
The plot is quite complex so I won't describe it in detail. Smiley has his suspicions and slowly but surely he gathers all the information he can to get to the bottom of the mystery. He is the ultimate "low key" spy, gleaning information from subtly interviewing people. Gary Oldman is excellent as the world-weary but still determined Smiley.
For me one of the best things about the movie is that it shows that despite the Cold War being apparently between two rival political philosophies the individuals involved are motivated by such basic emotions as lust, jealousy, greed and ambition. This applies even to Smiley himself.
While Smiley is the main character the supporting cast including Colin Firth, Benedict Cumberbatch and Toby Jones are outstanding. Swedish director Tomas Alfredson does a great job in bringing to life the grey paranoid world of the Cold War. He distills the novel into a slow moving but very satisfying movie.