A U.S. Marshal is unjustly convicted and sent to a notorious Arizona prison.

In a small Arizona town, in 1897, outlaw Abby Nixon reluctantly warns former marshal Billy Reynolds, who once spared her life, that two of her cohorts, Bud and Cole Gorman, are coming to kill him. Unarmed because of strict, new gun laws, Billy borrows a pair of six-shooters and meets his assailants head on. After a bloody gunfight, Bud and Cole lay dead, and Billy is arrested for murder. Despite pleas of self-defense, Billy is found guilty and sent to Devil's Canyon, a tough prison in the scorching desert. Although Warden Steve Morgan sympathizes with Billy, head guard Captain Wells is antagonistic, as Billy once slugged him for roughing up a suspect. Billy is also threatened by Bud and Cole's revenge-hungry brother Jesse, who is incarcerated at Devil's Canyon with fellow outlaws Red and Joe. One day, Abby, who is Jesse's sweetheart, arrives at the prison, having been found guilty of armed robbery. To protect Abby from the male prisoners, the warden assigns her to a room in the prison hospital, and Abby becomes Dr. Betts's assistant. Later, during dinner, Jesse throws a knife at Billy, and a brawl erupts. Both men are sent to the hospital, where Jesse and Abby enjoy a brief reunion. Abby then must tend to Billy's wound and treats him rudely, despite their mutual attraction. The next day, while working on a road gang, Jesse accuses Billy of slaughtering his brothers and tries to force him to reveal how he knew they were in town. That night, Abby bribes a guard to bring Jesse to see her at the hospital. There, Abby tells the sex-starved Jesse about the jailbreak she is planning and convinces him to wait until he is free to exact revenge on Billy. Abby then tries to talk Billy into participating in the escape, but he refuses. When Abby next meets with Jesse, he babbles incoherently about the escape, worrying Abby. Soon after, an informer alerts Wells and the warden about the breakout, and the warden subtly warns Abby not to take her chances with a brutal killer like Jesse. During a search of Billy's cell, Wells discovers two knives, surreptitiously planted by Jesse, and sends him to solitary confinement. That night, Abby hides some guns, which an outside cohort had shipped to her among some medical supplies, in the examination room. Dr. Betts, who believes that Abby and Billy belong together, then brings Billy to see her, and while changing the bandage on Billy's knife wound, she once again talks to him about joining the breakout. When Billy refuses, stating that he wants her to lead a decent life, Abby becomes emotionally conflicted. After Billy returns to solitary confinement, Red and Joe feign heat stroke so that they will be sent to the hospital. Abby arms them, and the escape begins. Red and Joe take three guards hostage and open Jesse's cell. The demented Jesse holds the warden at gunpoint and, declaring his intention to turn all the convicts loose on the nearby town and blow up the prison, demands access to the prison's dynamite. When the warden refuses, Jesse shoots Wells in cold blood. Now fearful of Jesse, Abby steals the key to Billy's cell and frees him. As the liberated convicts create havoc in the prison, Jesse discovers Billy's empty cell and slaps Abby for betraying him. Jesse then begins to hunt for Billy, but armed with one of Abby's guns, Billy is able to outmaneuver Jesse and his men. Having taken over the machine gun in the guard tower, Billy shoots Jesse and sprays enough bullets over the other convicts' heads to scare them back into their cells. After Abby runs to Billy's open arms, the grateful warden predicts that two pardons may be in store for them.