Lona Cohen, spy and courier for KGB agents in New York, is in Santa Fe to meet with Ted Hall.
However, after several attempts to meet Hall, he's a no-show—Hall, it seems, has gone MIA.
Fuchs, on the other hand, manages to convey to Gold complete descriptions of their plutonium bomb design, as well as a technical description of the uranium bomb used on Hiroshima.
Fuchs makes Gold promise they'll always be friends, and they split a BFF broken-hearts necklace as a keepsake of their love for each other. (Just kidding—but Fuchs does express his hopes that someday they can meet openly as friends and discuss less horrifying subject matter.)
Ted Hall knows he's missed a bunch of meetings with his courier, but he just couldn't get away at the time.
He finally meets up with Cohen, and Hall gives Cohen his final report.
Cohen has a really close call with FBI agents and soldiers inspecting her luggage at the train station, but some quick thinking and ingenuity allow to her get away with her precious information.
Both Fuchs's and Hall's reports make their way to Leonid Kvasnikov, the guy in charge of the KGB's New York offices, and he concludes that they're legit and really thorough.
This information allows the Soviets to begin truly building their own atomic bomb by October—and it's an exact copy of the American bomb.