Michael Lanyard (Warren William), known as "The Lone Wolf" in his safecracking days, retired from this line of work some time ago. But Mr. Gregory (Ralph Morgan), an international spy, and his thugs, are anxious to put him back in business to steal the top-secret plans for the Palmer anti-aircraft gun from the U.S. War Department. Lanyard turns down the job. One of Gregory's men does it himself -and sets up Lanyard to take the fall. However, the crooks find that they have only obtained part of the plans. They still need Lanyard's help to get what is missing.
A beautiful brunette named Karen (Rita Hayworth) lures Lanyard away from his girlfriend, Val (Ida Lupino), to Palmer's laboratory to crack the scientist's safe. Lanyard outwits them, takes the papers they're after, leaving for them a useless set of baby carriage blueprints. Evidence left at the lab and at the War Department incriminates Lanyard. He decides that the only way to clear his name is to return the plans in their entirety to the authorities.
Lanyard leaves his set of papers with Val's father, Senator Carson (Brandon Tynan), and sets out to retrieve the plans stolen from the War Department from Gregory. He sneaks into Gregory's home and takes them, believing now that he has a complete set. Meanwhile, Gregory's men realize they were fooled at the lab. Lanyard returns home to find that the Senator was strong-armed into relinquishing his section of the plans. Police are after him. Lanyard has got to work fast to get all the documents together.
With Lanyard making things increasingly difficult for them, Karen arrives on Gregory's behalf to try and bring him into their organization. Lanyard refuses. His daughter, Pat (Virginia Weidler), then chooses the wrong time to play detective. She stows away in Karen's car and lands in Gregory's home. Lanyard soon arrives there for Pat. Then Val turns up as well, but when her rescue mission fails, she winds up locked in a basement.
Just when things look hopeless, Lanyard comes up with an idea. With Pat's help, he recovers Gregory's half of the anti-aircraft gun plans, then saves Val. In search of Lanyard, the police track him down to Gregory's home. There they arrest Gregory, Karen and the gang, and Lanyard is proven innocent.