The young Joan Smith (Sophie Cookson) is studying physics at Cambridge University in 1938. She becomes involved with Communists and radical politics through her friends, Sonya (Tereza Srbova) and Leo Galich (Tom Hughes), German Jews. Sonya is an orphan and stays at the same hostel as Joan. William Mitchell is also a student at Cambridge.Sonya and Leo Galich are cousins, but the former grew up with the latter after she was orphaned and their relationship is more like that of a brother and a sister. Through Sonya, Smith meets and falls in love with the intense intellectual, Leo. They attend communist meetings and watch their propaganda movies on campus. Leo is a Russian, who escaped to Germany and was prosecuted for being a Jew. he then made his move to Britain and joined the communist movement there.Leo is a passionate orator and uses compelling argument to further the cause of communism at Cambridge. They sleep together just before Leo leaves for the USSR for 3 months for a research project. WW II breaks out. When Leo returns he is arrested, as he is a German as per British records. He in interned on the Isle of Man. Sonya went to Switzerland.Joan is recruited to work for the wartime Tube Alloys project (in 1941) to build an atomic bomb for Britain and meets the scientist, Max (Stephen Campbell Moore). Joan is a natural science student and is able to give intelligent ideas like using a centrifuge to separate the U235 isotope from the U238 atoms. Max gets attracted to Joan. Leo tries to recruit Joan to spy for the Soviet Union (HE knew Joan was working on the bomb, and asked her designs and research documents), but she rejects his appeal and ends her relationship with him, accusing him of using her.Joan travels to Canada with Max with his plan of producing a Plutonium fast breeder reactor, which is approved by deputy PM Clement Attlee (Robin Soans). Joan falls in love with Max and they have sex on the ship, but their relationship ends when Max tells her that he wants Joan as his wife, not his mistress, but, because of Britain's strict divorce laws, he is unable to divorce his wife. In Canada, Max works with Prof Kierl (Stephen Boxer). Max is in favor of sharing the research with the Russians as he thinks of them as allies, but the Canadians are set against this idea.During a tour of the University of Montreal, Joan is met by Leo again. Leo was in Canada, trying to get Canadian intelligence through to USSR. Leo says USSR has to know about the bomb as the Allies are going to do something dreadful with it. US tests the bomb in New Mexico, 108 long tons of Uranium producing 22 KT of TNT power. Max is also sure now that Allies wont want Stalin having a weapon like this at his disposal.In 1945, Joan is appalled by the atomic bombings of the Japanese cities of Hiroshima and Nagasaki, and is frightened when it is suggested that Britain develop its own atomic bombs to possibly use against the Soviet Union. Joan contacts the Galiches to provide information about the British nuclear program to the Soviet Union. Sonya gives Joan a miniature camera to sneak into her office and take pictures of everything.In an atmosphere of Cold War suspicion and paranoia, Joan finds her work increasingly difficult. Her office is raided by the police after Kierl was discovered to be a Russian agent. Joan manages to sneak the camera out by hiding it in a box on sanitary towels. After she again accuses Leo of using her (he asks Joan to come with him to the USSR), he dies, apparently a suicide, although it is later suggested he was murdered. Sonya flees Britain and Joan learns that Sonya's child was Leo's (when she goes to Sonya's house and finds evidence that Leo was with her this entire time). The revelation of this relationship between the Galiches adds to Joan's sense of betrayal, and she returns to Max.After the Soviet Union explodes its first atomic bomb in 1949, Max is arrested by Scotland Yard and is charged with espionage for the Soviet Union. Joan visits Max in prison where he informs her that his wife has agreed to a divorce. Joan in turn tearfully confesses to Max that she provided the intelligence that led to his being charged, but he forgives her. Joan blackmails Sir William Mitchell (Freddie Gaminara), a high-ranking diplomat who is really a Soviet spy, to help Max else she will reveal that William is gay (she has photos from their Cambridge days). As a result, Max is released from prison. Joan and Max go to Australia, but return to Britain at some point.In 2000, Joan (Judi Dench) is arrested and charged with espionage. She is interrogated by two detectives from Scotland Yard, whom she accuses of misunderstanding her life, but gradually concedes that she did provide information to the Soviets. The tabloid press goes into a "feeding frenzy" and vilifies her as a traitor, calling her "Red Joan". Her lawyer son, Nick Stanley (Ben Miles), first agrees to defend her but then disavows her when he learns that she did provide intelligence to the Soviet Union. Finally, Joan is able to convince him that her actions were motivated only by the desire to stop nuclear weapons being used again and he agrees to defend her, standing by her as she faces the tabloid journalists outside her home. The story was about Melina Norwood, exposed to be a Soviet Spy in the 1980s.