On 10 May 1916, during the First World War, British Captain and novelist Edgar Brodie (Gielgud) returns home on leave, only to discover his obituary in the newspaper. He is brought to a man identifying himself only as "R", who asks him 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), a fake death, and the assistance of a killer known variously as "the Hairless Mexican" and "the General" (Lorre), though he is neither bald, Mexican nor a general.