Mourning the end of a love affair, Judith Farrow leaves London, England, for a vacation in Barbados. There, she meets Feodor Sverdlov, a Russian intelligence agent working for the KGB, who asks her to join him for lunch one day. Sverdlov admits to being a Russian attaché and says that he has been stationed in Paris, France, for three years. Judith responds that she is a personal assistant to Sam Neilson, a British government official. A few days later, British agent Jack Loder learns that Sverdlov has been spending time with Judith in Barbados, and is convinced the relationship is a tactical move. British agents begin following the couple as they sightsee. At a museum, Judith views a seed from a tamarind tree that looks like a human head, and reads that a slave who was wrongly convicted and hanged from the tree announced before his death that the tree would vindicate him. Fascinated, Judith says she wants to find the same tree that produced the unusual seed. That night, Feodor asks Judith if she is afraid to make love, and she reveals that her husband died in a tragic car crash and her subsequent affair with Richard Paterson, a married British agent based in Paris, ended badly. Feodor admits that he is married, but doesn’t desire to see his wife anymore and no longer cares for the Socialist revolution. The next day, Judith and Feodor walk along the beach, and Feodor expresses his hopes that she will forget about Paterson and think of him instead. Before she leaves for London, he gives her an envelope to open on the plane, and Judith later discovers a tamarind seed inside. At a golf course in Paris, British agent Fergus Stephenson questions Paterson about his erstwhile affair with Judith, noting that Judith is now a security risk due to her relationship with Feodor in Barbados. When she returns to her apartment in London, Judith is met by Jack Loder and George MacLeod, British spies who interrogate her about Feodor. Though she insists that she formed a genuine friendship with the Russian agent, Jack says Judith must inform him the next time Feodor makes contact. In Russia, Feodor finds out that his assistant has been sent home to rest, as his superior, General Golitsyn, determined the assistant was under stress. Feodor tells Golitsyn about Judith and suggests he might be able to recruit her as an informant. Soon after, Feodor meets Judith in London and explains that he lied about recruiting her so that he can visit without looking suspicious. At a nightclub, Feodor and Judith run into Dimitri Memenov, a delegate of the Russian embassy in London. The men speak Russian, and, after Dimitri leaves, Feodor conveys that he is sending Dimitri to Russia to check on his former secretary. Feodor explains that he has grown suspicious that the secretary was sent away so that a new secretary could spy on Feodor. Later, Feodor tells Judith that he cannot divorce his wife because she is from a powerful family and a divorce would be politically dangerous. That night, Dimitri Memenov’s English girlfriend, Sandy Mitchell, visits Judith at home with a message from Dimitri warning Feodor to stay in Paris and not return to Russia as the KGB will try and persuade him to do. At the Stephenson’s home, Margaret confronts her husband after finding a tiny camera hidden inside his cigarette lighter and deducing that he is an informant for the Russians. Stephenson admits guilt, saying he has developed communist leanings. Meanwhile, Judith flies to Paris to inform Feodor of Dimitri’s message. Though she is distraught by the warning, Feodor reacts calmly and the two travel back to London. There, at Judith’s apartment, Feodor meets with Loder, promising that he can expose an informant working for Russia within their unit in exchange for his own political asylum in Canada. Offering a hint, Feodor says that the code name for the man is “Blue” and pledges to hand over the “Blue file” when Loder arranges for him to fly to Canada. Loder announces that two Russian spies are outside, and Feodor suggests he and Judith take a walk to throw them off. As they stroll, Feodor says that he and Judith should take another trip to Barbados, and kisses her to prove to the spies that he is seducing her. At a party, Margaret learns from Paterson’s wife, Rachel, that someone in the unit is suspected of informing the Russians, and she immediately tells her husband, worried that he will be exposed. Stephenson suspects Feodor is responsible and visits his Russian contact the following day with the news. Meanwhile, Feodor goes to the Russian embassy in Paris to inform Golitsyn that he needs to stay in London despite a request from his wife to return to Russia. Insisting that Judith is on the brink of becoming an informant, Feodor says he only needs to see her a few more days. After leaving Golitsyn’s office, Feodor visits the embassy’s file room and steals an incriminating document regarding Stephenson from the Blue file. With the help of Loder and MacLeod, Judith and Feodor steal away to Barbados, planning to spend a couple days there before Feodor travels to Canada and delivers the Blue file to Loder. Once Stephenson informs him of Feodor’s plan, Golitsyn sends four KGB agents after Feodor. On their first night in Barbados, Feodor and Judith make love. The next day, Feodor suggests they take a swim as the KGB agents close in on their hotel room. While Judith changes into a bathing suit, the KGB agents throw explosives into the hotel room, causing a massive fire. Sometime later, Margaret reads a newspaper article announcing Feodor’s death. Loder tells Stephenson that the Blue file burned with Feodor, saying he is convinced that the Russians were tipped off. Back in Barbados, Loder visits Judith, who is convalescing, with slight burns on her face. Handing her an envelope containing a tamarind seed, Loder says Feodor is alive in Canada and explains that MacLeod saved him just in time. As he rides away with MacLeod, Loder says they will allow Stephenson to continue working as an informant while providing him bad information to pass along, a strategy that will eventually lead to Stephenson’s death when the Russians decide he is a double agent. Later, Judith travels to Canada, where she and Feodor are reunited in the mountains.