The businessman Greville Wynnes, starring Benedict Cumberbatch, lives an undramatic life. However, it is completely turned upside down when he is unexpectedly hired as a courier by the British intelligence service and the CIA. They believe he is a person who would never be suspected of taking with him secret informationnb during his business trips. Grenville is attracted by the exciting mission and doesn’t tell his wife. The trips initially feels harmless and Grenville eventually becomes friends with the counter-spy Oleg Penkovsky. Then he understands that they are both in a bad situation and that they have received information that could avert a nuclear disaster.