Old, owlish, bookish George Smiley, retired spymaster, is approached in dead of night to covertly journey through the archived past to reassemble the threads and events surrounding the capture and torture of a British spy (and the forced retirement of Smiley and his discredited mentor). Smiley's slow, methodical work (through back door interview, through deduction, and through anecdote and flashback presented to the viewer) confirms the existance of a 'mole'. He prepares a plan to flush out the person (hidden among several probable), and puts it in motion.
I recorded this film off PBS (6 hours on Beta!) over three nights in the early 80's. Very, very slowly, the story draws the viewer in as George Smiley peels off layers of deception to get to the hidden core. The dialogue tosses around terms like 'mole' and 'safe house', and slang for the intellegence trade, that adds British flavor to an atmosphere of sad menace. The story is well-crafted; the melancholy atmospheres suggest a drawing-room who-done-it mystery. A conscious effort to be patient is necessary to appreciate the author (and the unmatched Alec Guinness) as they untangle the threads of an inside-out puzzle linked to code names like 'Testify' and 'Gerald'.
Recommendation: Buy the DVD(s) and set aside 2-3 evenings to watch. Then watch it again to see the missed clues, many subplots, and to appreciate the strength of the ensemble. On the other hand, if half hour plot resolutions are your forte', this probably moves too slowly; consider another movie instead.