During the tense days of World War II, the story revolves around Harvey Leeds, who serves as the secretary for the esteemed British criminologist Sir Stafford Hart. Harvey finds himself caught in a whirlwind of espionage when he visits the Hart mansion in London. Just as he is about to leave to be with his fiancée, Pamela—who is working at an air raid shelter—Harvey stumbles upon a mysterious briefcase belonging to Kent Wells, Sir Stafford's assistant. Curious, he peeks inside the case and inadvertently draws the attention of someone nearby.
That someone is Anton Schugg, portrayed by Forrest Tucker, an air raid warden who is later revealed to be a spy. Without warning, Schugg strikes Harvey unconscious and places him into the back of his ambulance, effectively kidnapping him. Meanwhile, Kent Wells notices Harvey going through his briefcase and, upon reaching the upper floor, observes a suspicious individual attempting to open Sir Stafford's safe. The intruder manages to escape with critical government secrets, specifically plans for a revolutionary weapon called the beam detector. In the chaos, he drops a distinctive cufflink engraved with the letter "L."
The case quickly draws the attention of Scotland Yard, which enlists the help of two American detectives, Insp. Stephens and Inspector Dickens, both visiting London, to investigate the theft. The lead points to Michael Lanyard, known as the "Lone Wolf", a notorious jewel thief and master of disguise, who is believed to be behind the espionage ring. The police visit the Hart mansion to piece together the clues, and suspicion centers around the "L," linking Lanyard to the case.
An air raid soon strikes London, plunging the city into chaos. Amidst the rubble and destruction, the Lone Wolf crosses paths with his loyal valet, Jameson. During the attack, he heroically rescues George Barrow, an air raid warden, from being crushed by a collapsed building. The Lone Wolf and Jameson subsequently take refuge in the air raid shelter where Pamela is working. Desperate to protect the plans for the secret weapon, the thief attempts to conceal them, but Inspector Stephens arrives just in time and arrests him for espionage. However, during the chaos, the Lone Wolf manages to slip away, and Jameson orchestrates the recovery of the stolen plans.
In the aftermath of his capture, the Lone Wolf cunningly seizes a gun, takes Sir Stafford Hart hostage, and makes a daring escape. Outside, he encounters Anton Schugg, who offers him a deal; Schugg, working for German intelligence, wants to buy the stolen plans. Schugg hints that the espionage ring is led by Gustav Sossel, played by Kurt Katch. The Lone Wolf is then taken to a German hideout, where he discovers that Harvey Leeds has been captured and is being held prisoner by the spies. Although convinced he does not possess the plans, the Lone Wolf promises to retrieve them in a few days.
In subsequent developments, Kent Wells is uncovered as Kurt Weil, a German agent, who reveals that the Germans plan to transmit the stolen plans via radio from their hideout. Meanwhile, Sir Stafford's death in the air raid is reported, and Inspector Stephens uncovers a secret document revealing that the Lone Wolf is actually working undercover for British Intelligence, a revelation that complicates the investigation further. The Lone Wolf then strategically contacts the Hart family, drawing police attention away from the real spy ring and guiding them toward his own safe house.
As the plot thickens, Pamela follows the Lone Wolf in an effort to uncover the truth. The thief cleverly tricks her into calling Inspector Stephens, bringing law enforcement close to the spies' lair at the Blue Parrot Café. The police, believing they are about to make a major breakthrough, surround the hideout. Meanwhile, the Lone Wolf manages to escape this trap by disguising his movements in Barrow's ambulance, keeping the police at bay.
The climax unfolds at the hideout, where Pamela, the Lone Wolf, and the police are captured by the German agents. It is revealed that Kent is actually Kurt Weil, a German double agent working within the spy ring. As the enemy broadcasts the stolen plans on the radio, George Barrow arrives on the scene. Pretending to hold a stick of dynamite, Barrow forces the German spies to drop their guns. Optimistically, the Lone Wolf then clarifies that the plans were fake—an elaborate bait to trap the enemy. Using this deception, he successfully delivers the genuine plans to the police, thwarting the espionage attempt and saving Britain from a dangerous breach of national security.
Throughout the story, themes of heroism, deception, and patriotism intertwine, with the Lone Wolf operating in a gray area between criminal and hero, all in service of his country. This story highlights the tension and danger of wartime espionage, showcasing clever disguises, quick thinking, and the relentless pursuit of justice amidst chaos.
