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After retiring from the wrong side of the law, ex-jewel thief Michael Lanyard, known as The Lone Wolf, settles down to a legitimate job in Washington, D.C. One evening, Lanyard is abducted by gunmen and taken to the leader of an espionage ring, Spiro, who offers Lanyard $10,000 to pick the War Department's safe and steal the plans for a new anti-aircraft gun. When Lanyard refuses to accept the assignment, Spiro releases him, but not before stealing several cigarettes from him. The following day, the War Department safe is robbed and Lanyard's half-smoked cigarettes are found at the scene of the crime. As a result, Inspector Thomas, Lanyard's nemesis, suspects the former jewel thief of the crime and pays him a visit. After giving the inspector his alibi, Lanyard leaves for a cocktail party with Val Carson, the senator's daughter, who is determined to marry Lanyard. Soon after arriving at the bar, Lanyard is approached by Karen, one of Spiro's operatives, and lured to Spiro's house, where he is informed that the spies made off with only half of the plans that were supposed to have been stolen, and that he will be expected to retrieve the remaining plans, which are located at the home of an inventor named Palmer. Lanyard is led forcibly to Palmer's house, where he manages to elude his captors long enough to remove the real plans from the safe and replace them with a baby carriage. After Spiro's men find the carriage, Lanyard turns the real plans over to Val's father and goes to Spiro's house to recover the remaining plans. Later, when Karen fails in her attempt to buy the secret plans from Lanyard, she drives away. Unknown to Karen, however, Lanyard's daughter Patricia, an amateur detective, lies hidden in her car. While trailing Spiro, Lanyard is recaptured by the master spy, but is saved by Patricia, who creates a diversion by stealing the plans once again. A car chase ensues and ends when the police arrive and arrest the spies.