Michael Lanyard, the Lone Wolf, Warren William agrees to go to Alexandria to help the Allied cause during World War II. There, he and his valet, Llewellyn Jameson [Eric Blore], are greeted by his old friend, nightclub owner Johnny Booth [Sheldon Leonard]. Fritz [Lloyd Bridges] comes to drive him, supposedly to see Sir Robert Wembley [Frederick Worlock], head of the British secret service in the region, but Karl [Gavin Muir], the leader of a Nazi spy ring, intercepts them. Karl threatens to kill Jamison, whom he has kidnapped, unless Lanyard agrees to do some as yet unspecified work for him. Lanyard reluctantly capitulates, and the two men are released. After they depart, Karl reveals to Fritz that he expects the Lone Wolf to try to trap him, but that is all part of his plan. When Lanyard meets with Wembley, the spymaster makes clear that he does not want an amateur's help, but reluctantly allows the Lone Wolf to play along in order to gather more information.
Complicating matters further, Lanyard and Jamison encounter Jamison's son Donald, [Robert Stanford], a British naval officer, and his fiancée, Valerie King [Ann Savage], in Booth's nightclub. Lanyard soon suspects that she is not all she appears to be. In Booth's private office, he also meets freelance spies or informers (more or less on friendly terms with Booth), who call themselves “Rembrandt” and “Cezanne.” Cézanne [Jay Novello] shows him that the lace King was knitting contains a secret message. When the two spies leave, Rembrandt shoots Cézanne; he dies in front of the nightclub, at King's feet.
Valerie returns to her hotel room, and Karl is waiting for her. She is one of his agents, currently extracting information from Donald for their real goal: the plans for the minefields and defenses of the Suez Canal.
Meanwhile, the Lone Wolf is approached by "Whistler" [Sig Arno], yet another unscrupulous man with information to sell. Whistler sells him lace that King had sent to a laundry; the hidden message indicates that whatever the Nazis plan to do is to be finished by midnight.
Karl visits Lanyard and gives him his assignment: break into a safe at British Intelligence and steal some documents. However, it eventually becomes clear to all that Lanyard's part is merely a distraction. The plans have already been stolen. Wembley orders the arrest of the Lone Wolf for treason, but Lanyard escapes.
He and Jamison head for the laundry. Along the way, they come upon the unconscious Donald. They revive him and take him along. Inside, they find secret rooms and overpower Karl. They also discover the body of Whistler and a clue, shards of a distinctive watch crystal, just like the one King has, microfilming equipment, and ashes of the defence plans. Lanyard deduces that the plans have been transferred to King's watch. When she telephones, Lanyard pretends to be Karl and learns that she is at the hotel. Before they get there, however, Rembrandt kills her and takes the watch to Karl.
Fortunately, Booth has an aircraft armed with machine guns. Lanyard pilots it, finds the speeding car taking Karl and Rembrandt to the submarine, and guns them both down.
