Harry Gold and Klaus Fuchs are clandestinely meeting for the first time on the freezing streets of New York City.
Fuchs is happy to be helping the Soviets, so he reveals to Gold as much as he knows about the Manhattan project.
Fuchs and Gold arrange to meet again—but never in the same place, and they'll also never wait longer than five minutes at any of the meeting places.
A month later they meet again. Fuchs details how he and the British team are separating U-235 from U-238.
Gold relays this information to his new contact, Anatoly Yatzkov (Semyonov was under too much heat from the feds), who then sends it directly to Moscow, where they're thrilled to finally have a physicist on the inside.
Fuchs and Gold meet several more times, and Fuchs gives Gold tons of technical details about the atomic bomb.
Suddenly in July, Fuchs disappears. Gold and the KGB are thrown for a loop.