A brunette woman is riding her bicycle down a country lane in rural England. She meets a man, links her arm in his, and takes a stroll. No, this isn't the beginning of some romantic comedy; it's an info hand-off between Ruth Werner, a KGB spy, and her contact.
Werner's contact is Klaus Fuchs (rhymes with "books"), a German-born physicist who's helping the British try to build an atomic bomb.
Fuchs has been a Communist ever since they were the only party to speak out against Hitler during his rise to power. Even though Britain has been his refuge ever since he fled Germany, his allegiance is stronger with the Communist Party—hence his desire to help the Soviets build their own bomb.
Fuchs has been slowly filtering data to Werner, which she then radios to Moscow from the safety of her home. When larger documents need to be transmitted, she meets with a Soviet contact in person.
Even though Fuchs is doing his best, it isn't enough. The real action is going down in the U.S., where the KGB is still struggling to establish agents.
Semyonov begs Harry Gold to keep his ears open for information regarding an atomic bomb.
Peter Ivanov, a KGB agent in San Francisco, is desperately trying to get close to Oppenheimer, whom the KGB knows is such a prominent physicist that he must be involved with building the bomb. Ivanov asks George Eltenton, a chemical engineer who knows Oppenheimer casually and is sympathetic to the Soviet Union, to hook them up. Eltenton doesn't know the scientist well enough, but they do have a mutual friend: Haakon Chevalier.
When approached by Eltenton, Chevalier agrees to talk to Oppenheimer about possibly spying for the KGB. (Still with us? Good.)
The perfect opportunity arises when the Oppenheimers (our boy Robert's married to a woman named Kitty now) invite the Chevaliers to dinner. In the car on the way there, Haakon is convinced Oppie will want to help them, but Barbara (Mrs. Chevalier) is adamant he won't.
Over martinis, Haakon mentions to Oppie that if he wanted to help the Soviets build an atomic bomb, he has the connections to do so. Oppenheimer is shocked that he'd suggest such a thing, protesting that it would be treason.
The news that Oppenheimer won't help gets back to Moscow.
Oppenheimer chooses not to tell General Groves about being approached by the Soviets, which ends up being a decision that will "haunt him for the rest of his life."