In 1981, Colonel Sergei Grigoriev (Emir Kusturica), a senior Russian official and French translator in the KGB, is disenchanted with the direction of the country under Brezhnev. He fears for his son and all other Russian youth who live under the cloud of nuclear war between Russia and the United States. So without seeking personal or financial gain, Grigoriev decides to pass on Russian intelligence to Pierre Froment (Guillaume Canet), a lowly French engineer working in Moscow. This young man finds it difficult to keep this secret life from his wife. The relationship between Froment and Grigoriev goes through many changes as they test each other and struggle with the differences in their temperaments and political perspectives. In one of the most touching scenes, the Frenchman delivers a package to Grigoriev containing champagne, the music of Queen (for his son), and a book of French poetry.

Farewell is based on a book by Sergey Kostine and directed by Christian Carion (Joyeux Noel, an incredibly moving film about the fraternization between enemies that occurred in the World War I trenches in December 1914). Although U.S. President Ronald Reagan considered the Farewell affair to be the most important espionage event of the 20th century, the Americans in the drama do not come across as trustworthy allies. Reagan (Fred Ward) and his advisors are seen watching a scene from John Ford's The Man Who Killed Liberty Valance and we sense the vanity and the cowboy independence in this administration as they make no effort to take up the Grigoriev's cause when he is caught by the Russians. Carion sees the Russian as a hero who single-handedly played an important role in changing the destiny of his nation and the world.