Richard and Frances Myles are preparing for their annual European summer vacation in 1939 when they are visited at their Oxford college by old friend Peter Galt, who has a seemingly simple job for them. Galt asks if they would start their holiday in Paris, meet a man there, and then continue their journey as he directs. But in the heightened atmosphere of pre-war Europe, nobody is above suspicion, in fact the husband and wife are being carefully monitored by shadowy figures. Soon the couple are racing across Europe and must use all their ingenuity to stay one step ahead of the enemy.
