Prideaux interferes with Smiley's arrangements for the mole; Smiley becomes department chief.
Smiley extracts vital information from Westerby and Esterhase, and realizes someone is following him.
Smiley learns Source Merlin is a fraud and visits Sam Collins, who reveals Bill Haydon as a suspect.
Smiley seeks information on Operation Testify and finds a clue linking Source Merlin directly to Karla.
Smiley accepts an assignment to flush the mole and begins investigating, with Guillam and Mendel as aides.
Retired agent George Smiley (Alec Guinness) learns one of his powerful former cohorts is a Soviet mole.