The Iron Curtain is based on the actual 1945 case of Soviet cipher clerk Igor Gouzenko, (Dana Andrews), who, after careful training, was assigned to the U.S.S.R. Embassy in Ottawa, Canada in the midst of World War II. Eventually, Gouzenko defected with 109 pages of material implicating several high level Canadian officials, outlined the steps taken to secure information about the the details of the nuclear bomb via numerous sleeper cells established throughout North America. The scandal that resulted when details of this case were publicized by American columnist Drew Pearson in early 1946 involved Canada, Britain and the United States.

Bostonites George and Catherine Apley live a proper life in a social circle. However, their daughter Eleanor's love for Howard Boulton and their son John's union with Myrtle threatens their home.

A British officer stationed in Ireland falls for the wife of an intelligence man.