On a crowded New York City subway train pickpocket Skip McCoy steals Candy's wallet. Unbeknownst to either it contains microfilm of top-secret government information. Candy was delivering an envelope as a final favor to her ex-boyfriend Joey, who told her that it contained stolen business secrets.
Government agent Zara had Candy under surveillance, hoping she would lead him to the top man in a Communist spy ring. He seeks police help identifying the pickpocket. Captain Dan Tiger has professional informant Moe Williams brought in. She asks Zara several questions about the pickpocket's technique, negotiates a price, and gives Tiger a list of names; Zara quickly identifies Skip from his mug shot. He tries to get Skip to give up the film, revealing its importance and appealing to his patriotism, but Skip denies he has it and laughs in his face.
Meanwhile, Joey persuades a reluctant Candy to track down the thief on her own. The trail leads to Moe, who is delighted to be able to sell the same information again.
Candy searches Skip's waterfront shack that night while he is out. Spotting only a flashlight when he returns, he knocks the prowler cold. When Candy comes to, she tries to get the film but fails.
The second time she visits she is puzzled when he calls her a "Commie" and "a Red" and demands $25,000 for the film. He reveals that he knows what is on it. Skip thinks she is only acting. Despite his rough treatment she finds herself falling for him.
When she returns to Joey his superior gives him a day to get the film back, and leaves him a gun. Candy finally realizes the truth. She turns to Moe for help, since Skip will not believe her if she warns him he is in danger. Moe tries to convince Skip to give the film to the government but fails. She goes home and finds Joey waiting for her. She refuses to give him Skip's address, and reveals she knows about the Communist plot. Joey shoots her dead.
The next morning Skip returns home to find Candy there. She blames herself for Moe's death, and is dismayed that he is still willing to trade with the Commies. When he starts to leave with the film, she knocks him out with a bottle and takes it to Zara and Tiger. Zara asks her to give it to Joey so he can lead them to his boss. Candy does, but Joey notices there's a frame missing. He beats her in an attempt to get Skip's address, then shoots her as she tries to flee. Rifling her purse, he finds the address.
Skip learns of Candy's beating and visits her in the hospital, comforting her with a romantic kiss.
Joey and his boss go to the shack, but Skip hears them coming and hides underneath. When Joey is ordered to deliver the film as is, Skip follows him to a subway station. He watches as an envelope is exchanged in a restroom, then knocks out the courier and chases after and beats up Joey.
Later, at the police station, Tiger predicts Skip will louse up his pardon and return to his criminal ways. A defiant Candy, arm in arm with her new love, says that's not the way it will be.
