Joan Stanley (Judi Dench) is a widow living out a quiet retirement in the suburbs when the British Secret Service places her under arrest. The charge: providing classified scientific information - including details on the building of the atomic bomb - to the Soviet government for decades. As she is interrogated, Joan relives the dramatic events that shaped her life and beliefs; her student days at Cambridge, where she excelled at physics while challenging deep-seated sexism; her tumultuous love affair with a dashing political radical (Tom Hughes); and the devastation of WWII, which inspired her to risk everything in pursuit of peace.