What is the main conflict in Tinker, Tailor, Soldier, Spy by John le Carre?

Tinker, Tailor, Soldier, Spy by John LeCarre is an exciting story about a mole who moves up to the highest levels of the British intelligence community. Based on the information he provided, the covers of some of the best agents were blown. They know he is there, but they don't know who or where and they have to find him. The safety and existence of the British intelligence community depends on it. How do they do it?