 While Smiley checks on other former colleagues for details of what is known about the mole, his protégé Peter Guillam plays Burglar Bill by sneaking into the Circus and getting corroboration of Tarr s story and the details of Operation "Testify". Smiley also focuses on source "Merlin", a British spy in the KGB, and his "Witchcraft" material upon which the present heads of the "Circus" owe their positions.