In 1960s London, salesman Greville Wynne (Benedict Cumberbatch) is recruited by MI6 to work as a go-between for an asset they have in Moscow. Under the guise of a businessman looking to open up western manufacturing for Soviet Russia, Wynne's true task is to act as an intelligence courier. With every Russian he meets potentially being an eye for the KGB, Wynne must continue to sell that he is nothing more than a simple businessman. However, when America and the U.S.S.R. find themselves on the brink of war due to the Cuban missile crisis, the stakes are raised as Wynne fully commits to the work in order to provide vital intelligence to avoid nuclear war.