Joan Stanley (Judi Dench) lives in contented retirement until her tranquil life is suddenly disrupted. The dark secrets she's been hiding since her university days have been uncovered by M15. She finds herself in their custody, accused of providing intelligence to the KGB. Cut to 1938 where Joan is a promising physics student at Cambridge University. There she falls for a young communist named Leo Galich (Tom Hughes) and through him, begins to see the world in a new light. After graduation, Joan secures a job in a weapons research facility where she comes to the realisation that the world is on the brink of nuclear disaster. Now Joan is forced to answer an impossible question: exactly what price would she pay for peace?