Farewell is set in Moscow during the Cold War in the early 80's.
Sergey Grigoriev (Emir Kusturica), a KGB colonel disappointed with his country's regime, decides to bring down the system. He makes contact with Pierre Froment (Guillaume Canet), a young French engineer posted to Moscow. The highly confidential information Grigoriev passes to Froment quickly attracts the attention of the Western secret services.
President Mitterand (Philippe Magnan) is alerted and decides to inform President Reagan (Fred Ward). The two heads of state decide to exploit this ultra-sensitive data transmitted by a mysterious Moscow source whom the French have baptised "Farewell."
A simple man, Pierre Froment hence finds himself embroiled in one of the most astounding spy cases of the 20th century. A case that is way out of his league and that soon threatens his life and the lives of his family.