A lawyer joins the FBI after his friend is killed and helps them catch gangsters he knows.
Attorney James Davis (James Cagney) throws a man out of his office, because he doesn't want to represent crooks. His friend Eddie Buchanan (Regis Toomey) knows Davis isn't getting business and suggests he also join the Federal Bureau of Investigation (FBI). Buchanan arrests gangster counterfeiter Durfee; but not allowed to carry a gun, he is shot and killed by Durfee. Davis signs up for the FBI. He sees Jean (Ann Dvorak) dance and kisses her good-bye. Racketeer Mackay (William Harrigan) paid $20,000 for Davis's legal education; but he approves of Davis joining the FBI, because he is quitting the rackets. Mackay tells his gang he is selling out.
The FBI has Jeff McCord (Robert Armstrong) train new men. Davis knows how to box, but Hugh Farrell (Lloyd Nolan) shows him jujitsu. Davis bumps into Jeff's sister Kay McCord (Margaret Lindsay). Bank guards are killed, and a gardenia is found. Davis says to check the fingerprints of Danny Leggett, and they match. The same rifle also killed Buchanan. Davis wants the job, but Farrell is assigned. Davis asks Kay for a picture before she goes to Chicago. Davis meets Mackay on a train, and Jeff asks about his mob connections. Davis tells Jeff he is going straight, and Jeff does not fire him.
Leggett's gang is robbing banks in a midwest crime wave. Farrell and police arrest Leggett, and a tip from Davis saves Farrell; but Leggett's gang gets him back by killing four policemen. The FBI gets new federal laws enabling them to carry weapons and make arrests. Farrell is killed. Jeff sends Davis to Chicago, and he traces money to a flower shop and finds Leggett. Davis climbs in the window and arrests Leggett. Kay worries about Jeff taking Leggett to prison and asks Davis to help. Jeff questions Jean, because she is married to Collins. Davis asks to talk with her alone. She says she left Collins, and Davis figures out Collins is with Mackay at his lodge. Collins (Barton MacLane) and his gang have Mackay tied up. Jeff, Davis, and the police surround them. About a dozen on each side shoot at each other. Tear gas is fired into the house. They run out; several are killed, and the car crashes. Mackay is shot too, and Davis is with him when he dies.
Davis resigns, turning in his badge and gun; but Jeff asks him to stay. Collins escapes and visits Jean, sending her out for whiskey. Davis and Jeff knock on her door, and Collins shoots Davis, escaping to a garage. Jeff tells Kay that Davis saved his life. Nurse Kay visits Davis and meets Jean. Kay is kidnapped, and Jean finds Collins at the garage with Kay gagged. Collins learns that Jean was at the FBI and shoots her. In the hospital a dying Jean tells Davis the address and kisses him good-bye. Davis goes there and shoots a gangster. Collins hides behind Kay but is shot driving away. Kay and Davis can now happily return to the hospital.
FBI Director J. Edgar Hoover approved this film that shows the federal agents adopting the violent tactics of gangsters to destroy them at their own game. Friends of Davis are killed, and he uses his knowledge of others to get them arrested or killed when they resist.