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Gang members Blackie Cross and Snooker try to pick up Judy Fox and her friend Pat as they welcome home Judy's brother, Marine Sergeant Marvin Fox. Blackie's rudeness starts a brawl, which results in Marvin being thrown in the brig and Blackie's arrest. Blackie is bailed out, then taken to the boss, Big Joe Kelly, who orders him to help in a deal involving foreign spies Murdock, Brill and Vogler. The spies want plans to the military's new aerial torpedo, and after Blackie bungles an attempt to steal the plans directly from Colonel C. B. Vincent and Captain Chester, Kelly orders him to join the Marines. Blackie engineers the shanghaiing of new recruit John Gordon in order to steal his enlistment papers, because Blackie's police record prevents him from enlisting under his own name. As the weeks pass, Blackie alienates the other Marines with his cynical attitude and hatred for authority. One night, Marvin catches Blackie as he is sneaking into the building where the top secret torpedo plans are kept. Blackie punches Marvin, and Marvin agrees to fight Blackie later that night, even though as a higher ranking soldier, he will be severly punished if he is caught brawling with a private. The pair are indeed caught and Marvin is demoted to corporal. Marvin does not tell Vincent the cause of the fight, however, and this selfless act shames Blackie, but Marvin refuses to accept his apology. Later, after being taunted by Snooker, who tells him that he is getting soft, Blackie steals the plans and hides them in his rifle butt. The next day, while Blackie and Marvin are unloading a cargo ship, the ship catches fire and Blackie rescues Marvin, who is trapped below deck. His heroic act brings respect from the Marines, and at a celebration party soon after, Blackie is touched by their new regard for him. The party is interrupted, however, when Blackie is taken away after Vincent learns the truth about his identity from the real Gordon. Blackie, unaware that he is being arrested, has a change of heart and asks Marvin to burn the papers in his rifle. Unknown to Blackie, Marvin is also arrested as he retrieves the plans, and once Blackie learns of the conspiracy charges against him, he assumes that Marvin squealed. Blackie is sprung from jail by Kelly, who blames him for Murdock's withdrawal from the deal. Kelly sends Blackie "for a ride," but Snooker saves him, then warns him that Murdock is planning to steal the torpedo itself. Blackie realizes he has become a true Marine and sets about to help protect the torpedo, which is being tested that day. Murdock's gang starts a forest fire to distract the Marines and then attacks the soldiers who have been left as guards. After the gang succeeds in stealing the truck containing the torpedo, Blackie sets the torpedo off, killing some of the gang, while the rest are captured. Blackie's heroics exonerate him and Marvin, and the grateful Marines help out Blackie when he is ambushed later by Gordon. Blackie has also proven himself to Judy, who happily accepts his attentions.