A government agent infiltrates a gang run by a mother-fixated psychotic.

Ruthless killer Cody Jarrett and his gang rob a train in California. During the robbery, Cody kills the engineers, and as one of the bodies falls, it activates a steam valve, badly scalding gangster Zuckie Hommell. Together with Cody's mother and his sexy, double-dealing wife Verna, the gang hides out in the mountains. Ma lovingly fosters Cody's criminal career and is the only one who can ease the blinding headaches that periodically immobilize him. She is his ally against Big Ed Somers, who is waiting for a chance to take over the gang and get Verna for himself. Cody decides to take advantage of an approaching storm to leave the hideout. After promising to send a doctor back for Zuckie, Cody surreptitiously orders gang member Cotton Valleti to kill him. Cotton only pretends to do the deed, however, and leaves a pack of cigarettes behind with his friend. The later discovery of Zuckie's scalded and frozen body, together with Cotton's prints on the cigarette pack, provide the Treasury Department with enough clues to link the train robbery to Cody's gang. Treasury agents, led by Philip Evans, come close to catching Cody, but thanks to Ma's warning, the gang escapes. Cody now creates an alibi for the murderous train robbery, a federal offense, by confessing to a robbery in Illinois that took place at the same time. Although Evans is aware that Cody is lying, he cannot prove it, so he sends for undercover agent Hank Fallon. Under the name Vic Pardo, Hank is sent to jail, where he plans to get close to Cody. Meanwhile, Big Ed takes advantage of Cody's absence to take over the gang. At the prison, Hank saves Cody's life when Roy Parker, one of Big Ed's associates, tries to kill him. After she hears about the attempt, Ma reassures Cody that she will take care of Big Ed. Cody begs her not to try, and his fears for her safety bring on a headache. Hank helps Cody, the way Ma did, and that night Cody reveals that he plans to escape. Hank conveys the escape plans to an agent who is posing as his wife, but on the day of the break, a newly arrived inmate reveals that Ma is dead. Cody goes berserk in the prison mess hall and is taken to the dispensary. There, he uses a smuggled gun to take the doctor hostage and, together with Hank, Parker and two other convicts, makes his escape. Outside, Cody kills Parker and then heads for Bakersfield to avenge Ma's death. When Verna learns of Cody's escape, she tries to sneak away, but Cody is waiting for her. Although Verna killed Ma, she tells Cody that Big Ed shot her in the back and offers to show him how to sneak past Big Ed's defenses. Cody kills Big Ed and then he, Verna and Hank join the rest of the gang. Copying the gimmick of the Trojan Horse, Cody plans to rob a payroll by sneaking the gang into a company inside an oil tanker. Meanwhile, Hank tries to tip off the police. While pretending to fix Verna's radio, he rigs up a signal that will locate the truck for the agents and then leaves a message on a gas station washroom mirror. The police track the truck to an oil plant in San Pedro and surround the area. Cotton spots them at the same time that one of the gang recognizes Hank as an agent. Cody then takes Hank hostage, but he escapes when the police throw tear gas into the plant. During the ensuing gunfight, all the gangsters are killed except Cody, who climbs to the top of an oil tank. Now completely insane, Cody yells, "Made it Ma, top of the world!" before exploding the tank with his bullets.