John Mills stars in this war story set after the fall of Tobruk in World War II. Two English army officers (John Mills and Harry Andrews) and two young nurses (Sylvia Syms and Diane Clare) are driving an ambulance through occupied North Africa to Alexandria. Along the way they pick up a South African officer (Anthony Quayle), and more than once avoid capture and death while crossing the German lines. However, as the South African officer begins to undermine their confidence, they gradually come to suspect him of being a German spy.