, deputy head of the British Secret Intelligence Service, is forced into retirement in the wake of Operation Testify, a failed spy mission to Czechoslovakia. Veteran British agent Jim Prideaux had been sent to meet a Czech general, having been told the general had information identifying a deep-cover Soviet spy planted in the highest echelons of British Secret Intelligence Service—known as the Circus, because of its headquarters at Cambridge Circus in London. The mission proves to be a trap, and Prideaux is captured and brutally tortured by the Soviets. Britain's chief spymaster, known only as Control, is disgraced and soon replaced for his role in Testify by Percy Alleline. Control's obsession with the Soviet mole was not shared by others in the Circus. On the contrary, the British believe they have a mole, Merlin, working for them in Moscow Centre, passing them secrets code-named Operation Witchcraft. Fears of a mole are revived when Ricki Tarr, a British agent gone missing in Portugal, turns up in England with new evidence backing up Control's theory whilst not identifying the mole. Control had narrowed the list of suspects to five men – Roy Bland, Toby Esterhase, Bill Haydon, Percy Alleline, and George Smiley – all of whom occupied high positions in the Circus. Knowing the Soviet spy is highly placed in the Circus, the British cannot trust the Circus to uncover its own mole or even to let its leaders know of the investigation; Smiley, who had been ousted along with Control while Control's other four suspects were promoted, is recalled to expose the mole. Under instruction from Oliver Lacon, the civil servant responsible for overseeing the intelligence services, Smiley begins a secret investigation into the events surrounding Operation Testify, believing it will lead him to the identity of the mole, who Moscow Centre has given the cover nameGerald. With the help of his protégé Guillam, who is still in the Circus, he gradually uncovers an ingenious plot, as well as the ultimate betrayal—of country, of the service and of friendship.
Shortly before filming began, Alec Guinness asked author John le Carré to introduce him to a real spy to aid him in preparing for his role. Le Carré invited Guinness to lunch with Sir Maurice Oldfield, who served as Chief of the British Intelligence Service from 1973–1978. During their meal, Guinness intently studied Oldfield for any mannerisms or quirks that he could use in his performance. When he saw Oldfield run his finger around the rim of his wine glass, he asked whether Oldfield was checking for poison—much to Oldfield's astonishment, as he was only checking how clean the glass was. The series was shot on location in Glasgow, at Oxford University in Oxfordshire, England, at Bredon School in Gloucestershire, England, and in London, including some of the Intelligence Agency scenes which were shot at the BBC.
Le Carré cited the series as his favourite film adaptation of his work, attributing this to his experience collaborating with Guinness. In a retrospective review in The New York Times, Mike Hale lauded Guinness's performance, "It’s conventional wisdom that Guinness’s performance is a landmark in TV history, and you won’t get an argument here, though if you’re watching it for the first time, you may wonder at the start what all the fuss is about." and cited the production's pacing versus current techniques; "Audiences used to the pace of the modern TV crime or espionage drama will need to reorient themselves." Retrospective reviewers favourably compared the series with the 2011 film version, also citing le Carre's praise of the original and referring to Guinness's performance.
