During World War I, British Captain and novelist Edgar Brodie (Gielgud) returns home on leave, only to discover his obituary in the newspaper. He is eventually contacted and asked to undertake a secret mission: to identify and eliminate a German agent on his way to Arabia to stir up trouble in the Middle East. Upon agreeing, Brodie is given a new identity, "Richard Ashenden", and the assistance of a killer (Lorre). When "Ashenden" arrives at his destination, he is surprised to find that he has also been provided with an attractive wife, Elsa Carrington (Carroll).
The film is loosely based on two of W. Somerset Maugham's Ashenden spy stories, "The Traitor" and "The Hairless Mexican".
This film features examples of:
- Agents Dating: Brodie/"Ashenden" and Elsa/"Mrs. Ashenden" end up falling in love during the course of their mission.
- Death Faked for You: The plot is kicked off by Brodie returning from leave only to discover his obituary in the newspaper; as it turns out, to be roped into a secret mission.
- Heroic Comedic Sociopath: The General, all the way. He works on the side of the protagonists but is a cold-blooded killer. This is best exemplified when a coded telegram informs them that Caypor the mountaineer, whom the General had just killed, is not their target. The General finds it very funny, Elsa is terribly distraught.
- Jumped at the Call: Elsa reveals she insisted upon the assignment for the thrill of it, much to the displeasure of Brodie/"Ashenden".
- Murder Is the Best Solution: When Caypor, the mountaineer who took a button important to the protagonists, decides to settle the issue with a bet of which one of them can climb higher on a nearby mountain, the General simply pushes the unsuspecting Caypor off a cliff.
- Non-Indicative Name: Peter Lorre's killer is known variously as "the Hairless Mexican" and "the General" (and is credited as the latter), though he is neither bald, Mexican nor a general.
- Overly Long Name: When passing as a Mexican General, Peter Lorre's assassin character calls himself General Pompilio Montezuma de la Villa del Conde del Ombu. He has surprisingly good comedic timing when delivering the name.
- Slippery MacGuffin: When Brodie/"Ashenden" and the General find a contact dead, they find in his hand a button, evidently torn off in the struggle. When they go to a casino to meet Elsa, the button is accidentally dropped onto a gambling table. Since it looks the same as his own chips, an experienced mountaineer assumes it is his and takes it. Ultimately downplayed in that once they recover the button it disappears from relevance in the plot.