George Smiley, deputy to the head of the British Secret Intelligence Service, has been forced into retirement in the wake of Operation Testify, a failed spy mission in Czechoslovakia.

Others in the British and American intelligence communities have been impressed with the information produced by Witchcraft, and Alleline and his team are regarded as a refreshing change from Control.

While on a routine mission Tarr had been approached by Irina, a low-level Soviet agent who claimed to know the identity of the mole and wanted to trade it for permission to defect.

Returning to London secretly, Tarr contacts Oliver Lacon, a senior civil servant who is the liaison between the Circus and the British Cabinet.

Before his ousting, Control had narrowed his 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.

With the help of his protégé, Peter Guillam, who is still in the Circus, Smiley begins a secret investigation into the events surrounding Operation Testify, believing it will lead him to the identity of the mole, whom Moscow Centre has given the cover name Gerald.

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.

[7][8] At the time of its release in the United States in 1980, Washington Post, television critic Tom Shales called the series "intricate and fascinating", and described its episodes as "six scintillating and delectable hours.

"[5] Writing in 2011, Guardian critic Toby Manning wrote that the series "is still unmatched in its evocation of the chilly, cynical cold war era".

"[16] Retrospective reviewers favourably compared the series with the 2011 film version, also citing le Carré's praise of the original and referring to Guinness's performance.