Ted Hall is writing a coded letter to Saville Sax using "book code" to establish a meeting date.
Sax tells everybody he's interested in attending the University of New Mexico (way to establish an alibi, Sax).
They meet in Albuquerque to exchange information. Sax relays a technical question the Soviets have for Hall, and Hall gives Sax two pages of handwritten notes, including everything he knows about the plutonium bomb.
Sax copies Hall's notes onto a newspaper with milk for ink. It's invisible once it dries until the paper is heated. Cool, huh?
Meanwhile, in February of 1945 in Europe, Sam Goudsmit and the Alsos team are right behind the Allied forces slicing through Germany. There they corner Walther Bothe, a German physicist, who isn't particularly forthcoming with information about Germany's atomic program.
They do learn that Werner Heisenberg and the rest of the German physicists have been moved to a town called Haigerloch.
At Los Alamos, Oppenheimer is even more stressed out than ever. His scientists are still puzzling out how to build a plutonium bomb, and they're not having much success.
They basically need to figure out how to create a perfectly symmetrical explosion inward onto chunks of plutonium—therefore creating a critical mass and starting a chain reaction.
Oppie puts George Kistiakowsky ("Kisty" for short) in charge.
Kisty decides he needs to have a hundred or so pieces of plastic explosives strategically placed and timed to all go off within a millionth of a second of each other.
This takes a ton of trial and error—and Ted Hall. He's in charge of figuring out what implosion looks like on a metal ball.
After a bunch of tests, they realize they've figured out how to build the plutonium bomb. Yay.
(But the guy that knows the most about it is a KGB informant. Boo.)
Saville Sax passes on Hall's information, but now the KGB is worried that it is disinformation—which could lead to quite a few heads rolling. They want Fuchs to corroborate Hall's report.
Gold is told by Yatzkov to contact Fuchs right away.
He meets Fuchs in Cambridge, Massachusetts (at his sister's house, once again), where Fuchs gives him his own report.
It will be impossible to get away from Los Alamos again (it's the final push toward building the bomb), so their future meeting place will have to be in Santa Fe.
Gold tries to offer Fuchs money for his report, and Fuchs is offended by the gesture.
Both of the reports from Fuchs and Hall make their way to Igor Kurchatov, the lead physicist at Laboratory Number 2 outside Moscow, who is like, "Hey. Sweet. They did all my work for me. Let's build us an implosion bomb."