A government agent infiltrates a gang; but then he falls in love with the gangster's sister.
In a war on gangsters Jeff Crane (Chester Morris) complains to the Warden (Lewis Stone) he got ten years. Jeff protests the bad food and is put in the hole on bread and water for 21 days. Jeff tells Sonny (Joseph Calleia) he will get guns sent in. They take them to the warden's office, change clothes with the prison board, and leave with them. While they change cars, Sonny is shot. Jeff goes to agent Duffy (Paul Kelly), and they plan to find Sonny's purple gang. Because of a $5,000 reward, Sonny kills the Negro Mose (Sam Baker). Jeff takes Sonny home. Sonny calls his gang together and orders the five men to rob a bank to distract the police.
Jeff's driving in the rain forces a bus into the mud, and Theresa O'Reilly (Jean Arthur) makes him drive them. Jeff barges in on Sonny's drunk doctor (Lionel Barrymore) and takes him in the car. They stop at a bar to get his medical instruments and learn the bridge is out. Jeff puts the doctor to bed, and Theresa gets him to take her to dinner. Jeff captures the undressed doctor. A bridge collapses in the rain, and Jeff finds Theresa in the car. When she kisses him for saving her, he drops her in the mud. Jeff wants to see her more later and learns Sonny is her brother. She learns that Jeff escaped with Sonny. The doctor says that Sonny needs blood and transfuses Jeff's blood.
Jeff tells Duffy about Sonny's planned hold-up and that Theresa is innocent. She begs Sonny to accept the farm he inherited until he slaps her. Jeff hits Sonny, who tells them both to leave. Theresa asks Jeff to give up crime. Duffy fires Jeff because of Theresa. Jeff gets drunk so that the doctor will take him to the hideout. Jeff picks up a machine gun and calls Duffy. Sonny and his gang arrive but escape from Jeff. Police move in, and a shooting battle begins. Jeff escapes and joins Duffy. The doctor drinks and is shot. Sonny's men run out and are killed; but Sonny escapes in a car.
Public hero Jeff and Duffy learn that Sonny had plastic surgery. Duffy tells Jeff to question Theresa, offering life imprisonment to Sonny; but she says she does not know where he is. A personal ad offering money draws Sonny to Theresa at a theater where she works. She sees police and warns Sonny, who runs, wounds Jeff, and is killed. In the hospital Duffy tells Jeff that Theresa is taking a train, and Jeff joins her in her compartment.
This drama also shows the trend toward making government agents heroes in the war on gangsters. Affection for crime figures changes to law officers as Theresa's feelings shift from her brother to Jeff.