What is the theme in The Honourable Schoolboy by John Le Carre?

Betrayal is a theme in the novel begins with a discussion about the Dolphin case and what might have gone wrong with it. However, the story begins long before the Dolphin case with the revelation that one of the British Secret Service's most loyal men was never truly loyal. Bill Haydon was found to be a traitor, secretly working for the Russians. Due to the fact that Haydon and the new head of the service were good friends, the head, George Smiley, is determined to fix some of the destruction Haydon leveled on Smiley's beloved Circus. Smiley begins investigating cases that Haydon may have falsely ended.