The story begins in Britain in 2000, when Joan (Judi Dench) lives a comfortable retirement life in the picturesque countryside. Although Joan looks like an elegant and dignified old lady, her true identity is actually a military spy. One day, an agent from MI5 knocked on Joan's door, breaking her peaceful life. Time goes back to 1930, when Joan (Sophie Cookson) was a physics student at Cambridge University. By chance, she met a Russian man named Lyon (Tom Hughes), and the two fell in love at first sight and soon came together. Under the influence of Lyon, Joan began to pay attention to the world's pattern, and found that history had come to a turning point. Military hegemony had already penetrated every aspect of her life, and the angry Joan decided to resist.