Stewart says Denniston is looking for a spy in their hut and Alan tells them the spy is actually John Cairncross. Stewart admits to knowing this before Cairncross even got to Bletchley; this is exactly why he placed them there – so they could leak whatever they wanted to Stalin since Churchill was too paranoid to share information with the Soviets. Cairncross is unaware that he is being used by them. Stewart says he needs Alan’s help to know what to leak to John and feed to the Soviets.