Counter-Espionage

During World War II, Harvey Leeds, working as a secretary for Sir Stafford Hart, campaigns for legislation to arrest all foreign agents in Britain. He visits the Hart mansion to gather information and is about to leave when he notices a briefcase belonging to Sir Stafford's assistant, Kent Wells. Harvey looks through the briefcase before heading out into the night, unknowingly getting involved in espionage activities.

Harvey finds and examines the contents of the briefcase, which belongs to Kent Wells, Sir Stafford’s assistant. His curiosity leads him to uncover plans related to Britain's secret weapon, the beam detector. As he prepares to leave, he is suddenly attacked and knocked out by Air Raid Warden Anton Schugg, who throws him into an ambulance.

Kent Wells notices Harvey going through his briefcase and later observes someone opening Sir Stafford’s safe. The thief escapes with the plans for the beam detector, dropping a cufflink engraved with the letter 'L'. This clue suggests the involvement of the notorious jewel thief Michael Lanyard, the 'Lone Wolf'.

Sir Stafford calls in Scotland Yard, and American inspectors Crane and Dickens are briefed by Inspector J. Stephens. They conclude that the letter 'L' points to Michael Lanyard as the likely suspect behind the theft of the secret plans. The investigation begins to link the theft to espionage activities.

During a London air raid, the Lone Wolf helps to save Air Raid Warden George Barrow from the collapsing building. After the raid, the Lone Wolf and his valet Jamison hide in an air raid shelter where Pamela, Sir Stafford’s daughter, works. The chaos offers him a chance to conceal the plans from the authorities.

Inspector Stephens arrests the Lone Wolf during a raid, but the plans are not found on him. Jamison retrieves the plans afterward, and the Lone Wolf manages to escape after grabbing a gun and taking Sir Stafford hostage during questioning. His escape complicates the espionage investigation.

The Lone Wolf encounters Schugg outside, who offers to buy the plans. Schugg is connected to the German espionage ring led by Gustave Sossel. The Lone Wolf is taken to the German hideout, where he learns that Harvey is a prisoner and that the Germans plan to transmit the plans to Berlin via radio transmitter.

The Lone Wolf is released with instructions from Schugg to retrieve the plans before the Germans transmit them. News reports falsely declare Sir Stafford dead in the air raid, fueling the cover story. The Lone Wolf secretly works undercover for British intelligence, making strategic moves to uncover the spy ring.

Pamela follows the Lone Wolf to his apartment after he leads the police away from the spy hideout. She tries to warn him about the danger, but both are eventually captured by the German agents at the hideout situated under the Blue Parrot Café. Their discovery puts them directly in contact with the spy ring.

It is revealed that Kent Wells is actually Kurt Weil, a German agent. The police, led by inspectors Crane, Dickens, and Stephens, are captured while tracking the spy ring. The Germans begin broadcasting the fake plans, but the Lone Wolf, with Barrow’s help, pretends to hold dynamite to distract the agents and reveal that the plans are false.

The Lone Wolf explains that the plans shown were bait to trap spies. He secretly hands over the real plans to the police, preventing the Germans from transmitting Britain's secret weapon and thwarting their espionage plot. The story concludes with the thwarting of the spy ring and the safety of Britain’s secret technology.