Three years after the end of WWII, Harry Gold falls in love with a chemist named Mary Lanning. This is wonderful for him, but he's totally torn as to whether he should confess to her that he's a Soviet spy.
Meanwhile, the Soviets are conducting their first test of their own atomic bomb, and it's a success.
The U.S. quickly finds out that the Soviets have exploded an atomic weapon, and their reaction is varied: Oppenheimer's like, "Duh," while Truman is baffled by their speed.
Robert Lamphere, FBI counterintelligence agent, figures out that someone probably told the Russians how to do it. Good thinking, Bob.
In 1949, American code breakers crack the code that the KGB used to send telegraphs to Moscow during the war. As they decipher the telegraphs, they realize someone was sending information from within the Manhattan Project.
The culprit: Klaus Fuchs. Busted. The FBI call MI-5 in to nail the dude.
There's a problem in convicting Fuchs, though: Even though the U.S. and Britain pretty much know it was him from the telegraphs, they don't want the world to know they've cracked the Soviets' code because then the Soviets will stop using it. So they can't use the telegraphs as evidence in order to convict Fuchs. It's quite the pickle.
So instead they lean on him. Hard. Over the next few weeks Skardon, an investigator with MI-5, continues to interrogate Fuchs about being a spy.
Finally, Fuchs buckles under the pressure and decides to confess everything.
Back in the U.S., Gold's paranoia about Mary Lanning discovering his duplicity actually ruins his relationship with her.
Gold reads about Fuchs's capitulation in the newspapers and realizes he's probably next.
Sure enough, Gold is questioned by FBI agents Miller and Brennan and then put under surveillance.
The FBI agents need physical proof that Gold is their spy, though they're pretty sure even without it.
He allows them to search his house, hoping they won't find anything incriminating (especially if he destroys all the evidence, first).