Michael Lanyard, an ex-jewel thief known as the Lone Wolf, travels with his butler, Llewellyn Jameson to Alexandria, Egypt to help the British government smash a German spy ring that is operating there. Lanyard is staying at a hotel owned by his old friend, Johnny Booth, and while drinking in the lounge with Johnny, Lanyard receives a note from Sir Roger Wembley, the British diplomat he is to meet later that night, directing him to a waiting limousine. The car transports Lanyard to a secret elevator garage that descends into a darkened room, where an unseen man informs him that Jameson is being held hostage and will be executed unless Lanyard agrees to steal some secret British documents. When Lanyard accedes to the spy's demands, he and Jameson are released. After the two are driven back to the hotel, Karl, the unseen man, informs his accomplice, Fritz, that Lanyard's assignment is a diversionary tactic to keep the ex-thief and his British allies occupied while the Germans steal the charts to the mine fields that defend the Suez Canal. Upon learning of Lanyard's meeting, Sir Roger, who resents the American's involvement in British affairs, reluctantly accepts Lanyard's proposal that he pretend to work with the spies to discover their mission. Later, Jameson receives a telegram from his son Donald, a lieutenant in the Navy, notifying him of his arrival in Alexandria that night. At Johnny's club, Jameson and Lanyard meet Donald and his fiancée, war correspondent Valerie King. When Valerie's purse spills open and a patch of crocheted lace falls out, Johnny becomes suspicious and has a vague recollection of meeting Valerie before. After retiring to his office, Johnny introduces Lanyard to Cezanne and Rembrandt, two mercenaries who have been sent by Sir Roger to aid the American in his mission. When Cezanne shows him a message he has intercepted from the Germans concerning charts of the location of the Allied army in Iran, Lanyard suspects that the Germans plan to invade the Mid-East through Turkey and expect him to steal the charts. After leaving Johnny's office, Rembrandt shoots Cezanne in the back. Valerie, upon returning to her hotel room, discovers that her passport is missing and panics, explaining to Donald that the document is a forgery. After Donald leaves to search for the passport, Karl emerges from the shadows. Valerie informs him that Donald has arranged a personal tour of the Admiralty for her and promises to pass along the layout to the spies. Lanyard, meanwhile, is strolling down the street when The Whistler, another mercenary, approaches him and reveals that Valerie delivered the lace to a laundry and has arranged to pick it up at midnight. Hearing The Whistler's information, Johnny remembers that Valerie was involved in a spy scandal with a French officer. Later, The Whistler returns to Johnny's office with the lace, and when they unravel it, Rembrandt deciphers a code directing Valerie to a midnight meeting with a submarine. Upon returning to his room, Lanyard is visited by Karl, who tells him that they have kidnapped Jameson and will execute him unless Lanyard steals a file from the vault at Naval Intelligence that night. At Naval Intelligence, Lanyard is in the process of breaking into the vault when a car carrying Jameson crashes into the gates. When Sir Roger arrives with news that the Admiralty has been broken into and a copy of the defense of the Suez Canal stolen, Lanyard realizes that he has been duped. Holding Lanyard responsible for the theft, Sir Roger orders his arrest, but Lanyard sets fire to the car, and in the confusion, escapes with Jameson. Determined to retrieve the charts, Lanyard and Jameson search the streets and find Donald lying unconscious in an alley. Upon regaining consciousness, Donald explains that he was following Valerie when someone hit him on the head. Continuing down the alley, the three men come upon the laundry and force their way in. There, they coerce the workers to divulge the spies's hideout. After tying up the workers, they descend into the basement hideout and are greeted by Fritz, whom they overpower. When they find The Whistler's dead body holding a broken watch crystal in the palm of his hand, Lanyard puts the crystal pieces together. Upon projecting the pieces through an ultraviolet projector, Lanyard discovers that a map of the Suez Canal has been microfilmed onto the crystal and deduces that Valerie must be wearing a watch bearing the identical plan on its crystal. Soon after, Valerie telephones and Lanyard, pretending to be Karl, tricks her into divulging her location at a hotel. Lanyard then calls Johnny for help, and Johnny agrees to meet him at Valerie's hotel room. Before they arrive, however, Rembrandt appears at Valerie's door and demands the watch. By the time Lanyard, Johnny, Donald and Jameson get there, the room is empty. When the clerk remembers seeing two men waiting outside in a roadster, Johnny offers Lanyard the use of an old training plane equipped with machine guns. Lanyard takes off in the plane and, after locating the car carrying Karl and Rembrandt, strafes it. The machine gun fire catapults the car off the side of the road, causing the watch to smash in the wreck. Later, at the club, as Johnny, Donald, Jameson and Lanyard celebrate their victory, Lanyard receives word of another assignment from Sir Roger, causing Jameson much distress.