Cocky Tommy Connors [Spencer Tracy] is sentenced for up to 30 years in Sing Sing for robbery and assault with a deadly weapon. His associate Joe Finn [Louis Calhern] promises to use his contacts and influence to free Connors, but his attempt to bribe the warden [Arthur Byron] to provide special treatment is met with disdain and failure. Connors makes trouble immediately, but the prolonged confinement in his cell begins to change his attitude. As the warden had predicted, Connors is glad to contribute some honest work on the rockpile after his period of inactivity.
However, Connors's determination to escape is unshaken. Bud Saunders [Lyle Talbot], a highly educated fellow prisoner desperate to be with his pregnant wife, recruits Connors and another inmate for a complicated escape attempt. However, the escape is scheduled for a Saturday, a day that Connors superstitiously regards as unlucky for him. He withdraws from the plan, forcing Saunders to take another volunteer. The warden is alerted to the escape attempt, and although two guards are killed, the attempt is foiled. Trapped, Saunders jumps to his death. His two accomplices are captured and returned to their cells.
Connors's girlfriend Fay Wilson [Bette Davis] visits him regularly in prison since his trial. On one visit, she admits that she has become close to Finn to encourage him to help Connors, but Connors tells her that she is only providing Finn with a reason to keep him locked in jail. Connors learns that Fay was injured in a car accident and is not expected to live. The warden grants Connors a 24-hour leave to see her, and Connors promises to return.
When Connors sees Fay, he learns that Finn was responsible for her injuries. He takes a gun from a drawer, but Fay persuades him to give it to her. Finn appears, expecting Fay to sign a statement exonerating him in exchange for $5,000 that he intended to give to Connors. Connors and Finn fight, and Fay shoots Finn. Connors flees, taking the gun with him, and Fay secretly slips the money into his pocket. Before he dies, Finn names Connors as his killer.
The warden is lambasted in the newspapers for allowing Connors to leave the prison. Just when the warden is about to sign a letter of resignation, Connors emerges. Connors is found guilty of first-degree murder and sentenced to death in the electric chair despite Fay's testimony that it was she who killed Finn. Connors comforts her before being taken to death row.