Harry Gold and Anatoly Yatzkov are planning Gold's trip to Santa Fe to meet with Fuchs.
Everything seems to be their standard plans, until Yatzkov proposes that Gold take a side trip to Albuquerque to get info from another source. Gold is like, "Aw, heck naw—that goes against all KGB regulations, and could jeopardize my meeting with Fuchs."
Despite that, Yatzkov is determined to have this guy confirm the info they're sending to Moscow, and he's not taking "no" for an answer.
The scientists as Los Alamos celebrate, and also feel like their work is done. The bomb was supposed to be used against the Nazis, so why continue?
But Secretary of War Stimson messages the scientists and encourages them to keep working with all due haste. They still have to defeat the Japanese.
Now Oppenheimer is wrestling with the debate over whether they need to test the plutonium bomb. They don't have enough uranium to test that one, but the plutonium bomb—which is far more complicated—would be an option.
Oppie wins the debate against Groves, and sets up a test site near the Alamogordo Air Force Base in New Mexico, and names it Trinity.
The work at Trinity is frantic—they're all rushing to set up the structure for the test, as well as a bunch of structures to house machines to measure the blast, and places for people to watch the blast as well.
Gold arrives in Santa Fe with plenty of time to buy a map and mark his meeting spot with an X. (Um, really? You're a super spy and you do something like that?)
Gold and Fuchs meet, and Fuchs hands over what he considers to be the worst he has ever done by giving information about the principle of the design of the plutonium bomb.
Their meeting complete, Gold heads to Albuquerque to meet this other corroborative source, code name Mr. Greenglass.
Oh wait, that's his real name: Greenglass is an army sergeant assigned to Los Alamos, and he doesn't know nearly as much as Fuchs does, but he does know enough to corroborate what Fuchs is giving the Soviets.
Gold gets home to New York City via planes, trains, and subway and gives all his information to Yatzkov.