An anonymous letter had accused Foreign Office man Samuel Fennan of being a communist.
George Smiley, assigned by the Secret Service to vet Fennan, had assured him he had nothing to fear.
No one believed this nonsense. But clearly something had rattled Fennan, for a day after the interview he killed himself. Smiley is told by his angry superiors to fix this mess.
The last thing anyone wants right now is a scandal. It is immediately obvious to Smiley, however, that the suicide story doesn't make sense. Piece by piece he uncovers a conspiracy - one originating in East Germany and led by a wartime ally of Smiley.
To expose it, he must turn a former friend into a deadly enemy ...