Richard Burton plays a tired British secret service agent who has been in charge of Berlin Section for the past nine years. He is sent on a mission by Control and Control's right hand man, George Smiley, to play defector for the soul purpose of undoing their opposite number, Mundt, the East German secret operations commander.
This is not a film in which the plot hinges on whether a spy can use his laser watch before an evil mastermind can gain world domination. John LeCarre renders a patient, highly intelligent portrait of intelligence agencies and the ethical ramifications surrounding innocent casualties and the betrayal of personal loyalties. Unlike James Bond/Bourne Identity Films, his stories are important, say something about ourselves, and resonate deeply. The execution of the trial scene near the end, when Alex realizes the horror of what he's actually been participating in, is masterful.
Why can't more films be of this caliber? There are plenty of us who enjoy real craftsmanship, taught yet subtle stories about genuinely interesting people. It seems like the right director could make them pay for themselves. Most of the movie takes places either sitting at tables or walking through the countryside. You would need no collapsing skyscrapers or stuntmen. Just a handful of talented actors and a good script.
I'd note that you get to see a young Robert Hardy, who later played Siegfried in All Creatures Great and Small and, most recently, plays Minister of Magic Fudge in the Harry Potter Movies.