The closing credits note the real-life Melita Norwood incident as having "inspired" the film. At the end of this movie, the reporters outside of Joan Stanley's (Dame Judi Dench's) home ask, "How much money did you get?" She answers, indignantly, "Nothing." In reality, Norwood stated, "I did what I did, not to make money, but to help prevent the defeat of a system which had at great cost given ordinary people food and fares which they could afford, a good education, and a health service." (New York Times report 13.9.99.) At that time, the U.K.'s newly elected Labour (Socialist) government under Prime Minister Clement Attlee (shown and played in this movie by Robin Soans), had introduced its first publicly (taxpayer) funded welfare state. On the first day of the new parliament, Labour members sang the socialist anthem the Red Flag.
This movie was very loosely based on Melita Norwood (the real Red Joan), who was the Soviet Union's longest-serving British spy, having done so for nearly four decades. As a longtime member of the Communist party and a secretary for the Non-Ferrous Metals Research Association in London (which was actually part of a secret nuclear weapons research project with the U.S. called "Tube Alloy"), Norwood copied nuclear secrets from her bosses' office and transmitted them to Moscow. Unlike what the movie's implications, she was a spy from World War II through the Cold War, retiring in the early 1970s, and in 1979 she traveled to Moscow to receive the Soviet Union's Order of the Red Banner.
Joan Stanley (Dame Judi Dench) is questioned by MI5 (Military Intelligence, Section 5). Formed in 1909 and named "Security Service" in 1964, this is a U.K. government agency that deals with internal security and counterintelligence on British soil.
The Manhattan Project, as mentioned in this movie, was an American project of research and development during World War II that produced the first nuclear weapons.
Early in the movie, Leo Galich (Tom Hughes) is arrested for being German. Hughes also played Prince Albert in Victoria (2016), a person of German descent who was reportedly unwelcome within the court of Queen Victoria for precisely that reason.