May 10 1916: Mourners attend the funeral of Brodie, a successful novelist and soldier who has supposedly been killed in action. The story of his death, however, has been propagated by the Secret Service to enable Brodie to take on an espionage mission, to pursue a German spy who is seeking to forge military alliances with Arab forces and prevent him from reaching Turkey.

Assuming the name of Ashenden, he is despatched to Switzerland, where he meets the womanising assassin the General, Elsa, another agent who is playing the part of his wife, and Marvin, an American socialite who has taken an interest in Elsa.

Ashenden and the General go to a church in a small village, where they are expecting to make contact with a German double agent, only to discover that the enemy has got to him first, and he is already dead. Their only clue is a button found in the victim's hand, presumably wrenched from the coat of his assailant.

Returning to their hotel, they discover that Elsa has left for the Casino with Marvin, and head off to join them. At the casino, they discover that Caypor, an English guest at the same hotel, appears to be missing a button of the same type. It also emerges that Caypor has a German wife, and that he was walking in the same village that day. Ashenden and the General hatch a plan to take Caypor with them to climb a nearby mountain, where he will be killed by the General.

On the mountain, Ashenden has second thoughts about the plan, but is overruled by the General, who carries it out as Ashenden watches helplessly through a telescope. Elsa, who is back at the hotel where she is charged with keeping Mrs Caypor occupied, becomes increasingly agitated as the reality of the murder dawns on her. Her distress grows when it subsequently emerges that Caypor was not their man, particularly at the General's response to this revelation.

Later, Elsa and Ashenden, who have by now fallen in love, resolve to resign their jobs, but Ashenden changes his mind when the General learns that German spies are operating out of a nearby chocolate factory, and an employee of the factory is prepared to reveal the true identity of their quarry. Leaving behind the distraught Elsa, Ashenden and the General head for the factory. They are spotted and pursued by police, but manage to evade them and meet their contact, who tells them that the real enemy agent is Marvin.

At the hotel, Elsa, now thoroughly disillusioned, pleads with the departing Marvin to take her with him to Greece. Assuming Elsa has discovered Marvin's true identity, Ashenden and the General head for the train station in pursuit. They meet Elsa, and realise that Marvin has boarded a different train, bound for Constantinople. The three follow.

Elsa pleads with Ashenden not to kill Marvin, but he insists that it is his duty. At the border, the train is boarded by a platoon of Turkish soldiers. In the corridor, Elsa is seen by Marvin, who believing her to be alone, takes her with him. As Ashenden and General pursue, the train is attacked by British planes. They corner Marvin in a carriage, but Elsa, using Marvin's gun, stands in their way. Just then the train is derailed by a bomb on the track. In the wreckage, Ashenden reaches out to strangle the injured Marvin, but is unable to do it. Marvin, gets hold of a gun and kills the General, then dies himself of his injuries.

Their mission successful, the Turks are defeated and Ashenden and Elsa - now officially Mrs Ashenden - resign from the service.