As the film begins, Control (John Hurt), the head of M16 (the British secret service, known colloquially as "the Circus"), sends a field spy (Mark Strong) into Budapest to talk a Hungarian general into defecting. The general knows the identity of the mole in the Circus who has been feeding secrets to the Soviets. Things, however, go horribly wrong, and Control's reign is ended.
Who is the mole? Brought back into the Circus following his enforced retirement and Control's death is Smiley, who narrows his search to four men at the top: Percy (Toby Jones), Bill (Colin Firth), Roy (Ciarán Hinds), and Toby (David Dencik) – all beautifully played.
The quartet of suspects is more like a cabal. Meeting in airless office rooms, the atmosphere thick with smoke and deceit and pumped-up bonhomie, we are left to think that any one of these men, or several of them, could be traitorous. The gifted Swedish director Tomas Alfredson, who made a splash with his new-style vampire movie "Let the Right One In," presents these agents as a species of night stalker. He doesn't overdo it, though. This is a horror film at its most cerebral – all feints and jabs and innuendo. With the exception of that flashy shoot'em-up centerpiece in Budapest, the film is almost totally devoid of large-scale action. It's a bloodless movie about a bloody business.