The story takes place in Britain in 2003. Catherine (Keira Knightley) is a government communications translator serving the British intelligence agency. One day, she received an encrypted email that revealed the conspiracy of the NSA to collude with the British government to invade Iraq and start a war. Catherine lives a stable life that everyone envy, with a good salary and a promising future. This email made her entangled in whether to turn a blind eye or bravely stand up to report and expose. In the end, the sense of justice defeated the selfish desire. Catherine and lawyer Ben (Ralph Fiennes) jointly made the email public, which soon aroused public excitement. Immediately afterwards, Catherine was accused of treason for violating the National Secrets Act.