Bartholomew "Barley" Scott-Blair, the head of a British publishing firm, travels to Moscow for business commitments. While at a writers' retreat near Peredelkino, he expresses hopes for easing tensions with the West, an idea overheard by the enigmatic Dante, who insists that Barley must promise to do the right thing if a chance presents itself.
Months pass, and when Barley is unreachable at a trade show, a young Soviet woman named Katya Orlova approaches publisher Nicky Landau with a manuscript she wishes to deliver to him. Landau takes a peek at it and forwards it to British intelligence. The document reveals the Soviet Union's nuclear warfare capabilities, leading to an investigation that identifies Dante as the famous Soviet physicist Yakov Efraimovich Saveleyev, the author of the manuscript.
British intelligence agents track Barley to his holiday residence in Lisbon, questioning him about his associations with Katya. Realizing he is as clueless about the situation as they are, they decide to engage him to aid their mission. Meanwhile, MI6 has intel that the manuscript is equally critical to the CIA, with both organizations eager to enlist Barley for their own purposes. A British agent, Ned, provides Barley with some rudimentary spy training.
In a bold move, Barley travels back to the Soviet Union to seek out Dante and validate his role as an informant. His encounter with Katya proves pivotal as he becomes infatuated with her. Through her guidance, he discovers that Dante indeed is Yakov and reassures Katya that he is not a spy.
As the British intelligence agency conducts the operation's initial phase, they keep the CIA in the loop regarding their discoveries. The CIA team, led by Russell, expresses concerns about the revealing nature of the manuscript, which depicts a chaotic Soviet nuclear missile program, hinting at a futile arms race fueled by the United States.
Katya arranges to meet Yakov, meticulously ensuring they aren’t being surveilled. Barley conveys that the manuscript is now in the possession of British and American authorities. Feeling deceived, Yakov contemplates betrayal, but Barley convinces him that there is still a chance for the manuscript to be published, receiving another volume in return, alongside his assurance of support for Yakov’s scientific endeavors.
Russell's superior, Brady, and U.S. military officer Quinn interrogate Barley to affirm his allegiance as they speculate on the truthfulness of the manuscripts. Despite showing ideological support for glasnost, Russell acknowledges that this stance could jeopardize his connections in the arms industry who thrive on continued hostilities.
Driven by the manuscript’s discoveries, MI6 and the CIA compile a "shopping list" of inquiries for Dante. Ned, the operation’s handler, begins to suspect that Barley may be compromised, but the combined teams remain committed to their strategy.
Returning to the Soviet ground, Barley reveals his feelings for Katya, indicating he is, indeed, an operative. Katya, however, expresses concern for Yakov's well-being, suspecting he may be under KGB control. She provides Barley with Yakov’s Moscow address.
Under strict surveillance from British and American intelligence, Barley makes his way to Yakov's apartment for their meeting. Ned suddenly deduces that the Soviets are likely aware of the operation, intending to steal the shopping list to glean intelligence from it. Although he believes that Barley may betray their cause, Russell opts to proceed with the original plan. Their brief expected meeting stretches into seven hours, leading Russell to recognize that he miscalculated the situation.
Barley sends a note to Ned, disclosing that during a prearranged phone call to Katya, Dante used a code word indicating he had been compromised by the KGB, with Katya’s life hanging in the balance. In an act of desperation, Barley facilitates the transfer of the shopping list to the Soviets, bargaining for the safe passage of Katya and her family to the West. He admits to Ned that his choices may be questionable but insists, > "You shouldn't open other people's letters."
In the finale, Ned pays a visit to Barley, who has returned to Lisbon to await Katya's arrival. When her ship finally docks, Barley stands at the shore, ready to embrace Katya and her family, eager to embark on a fresh start together.
