By early 1943, the Soviet army has finally halted the Nazi progression across Russia, which is a huge accomplishment, but now they're begging the Allies to launch an invasion of German-held Western Europe so that Hitler won't have to fight on two fronts.
Roosevelt and Churchill tell Stalin to cool it, they're busy fighting Italy and Japan. They also sign a special pact not to reveal the atomic bomb plan to the Soviets.
The CIC (Army Counter-Intelligence) is determined to keep the atomic bomb a secret, and they're still worried about Oppenheimer.
Clueless Oppie keeps making them nervous by meeting with old comrades like Jean Tatlock—an ex-girlfriend and member of the Communist Party.
Is Oppie having an affair with Tatlock? Or is he sharing secrets with the Soviets about the bomb?
Groves refuses to remove Oppenheimer from the project (as his superiors wish him to do) because he believes the guy is critical to their success in building an atomic bomb before Germany does.
However, they do warn Oppenheimer not to associate with his "questionable friends" anymore, and inform him that whenever he leaves Los Alamos he'll be closely watched.
This is when Oppie decides to come clean about the time Chevalier had asked him to share secrets with the Soviets, and instead of reassuring Pash (a CIC agent) that he's trustworthy, this alarms him.
From the other side, even though the FBI and CIC suspect Oppie is spying for the KGB, numerous memos and cables from the time show that while the KGB never gave up trying to cultivate Oppie as an agent, they also never made any progress on that front. Ah, twenty-twenty hindsight.
Meanwhile, the Soviets have built "Laboratory Number 2" outside Moscow for their atomic bomb building site, and put physicist Igor Kurchatov in charge.
Kurchatov is pretty much relying on the information Klaus Fuchs has been sending from Britain. His spies have provided just about everything except for technical material on bomb design—so now the KGB needs to focus on getting that from America.
The KGB get a huge break in late 1943 when Oppenheimer requests new top physicists from Britain. Guess who they send him? That's right: Klaus Fuchs.
In light of this development, Semyonov contacts Harry Gold and tells him to forget everything he's been working on because now he has a new project and it's crucial.