Escaped convict and incorrigible criminal Lou Gannon is cornered and wounded by undercover police detectives in a New York City subway stairwell. While awaiting word of Gannon’s condition, the chief of detectives reflects on Gannon’s imprisonment: After a number of unsuccessful attempts to convict Gannon, he is found guilty of armed robbery and sentenced to twenty years at a maximum security prison. Upon his arrival at the prison, Gannon meets old acquaintance Doc, fourteen years into a long sentence. Doc tells Gannon that the violent Ed “Bugsy” Kyle, another former crony of Gannon’s, controls the prisoners. Bugsy and his gang welcome Gannon, and Bugsy readily gives up his top position when Gannon reveals that he has a sophisticated breakout plan with organized outside assistance. Soon after, Gannon is assigned a cellmate, young Rudy Hernandez, a first- time offender who was unwillingly involved in a holdup and shootout. Despondent over his five-year sentence, Rudy confides in Gannon that he is determined remain on good behavior in hopes of getting an early parole. Gannon takes a paternal interest in Rudy, advising him to avoid the stern guard captain, Starkey. Rudy is pleased when he is assigned work duty in the prison garage. Several months later, Gannon has manipulated his way into a job assignment in the supply office, where he assists in monitoring prison supplies. One day, Bugsy, who stocks the warehouse, tells Gannon of a new supply shipment. A member of Bugsy’s gang, Al Carey, delivers supplies to and from the warehouse and arranges to pickup several blocks of soap from the new shipment, which he then takes to Gannon to check in. Gannon meets Al in the stockroom, along with Doc, Bugsy and the rest of Bugsy’s small gang. There, the men dig into the soap blocks in which are hidden gun parts that they have been accumulating for several weeks. The men have constructed three full machine guns from previous shipments, which they have hidden in a storeroom crate, and are now awaiting the barrel for a fourth gun. Although anxious about the risky delay between shipments, Gannon rejects Bugsy’s suggestion to transfer the weapons to a new hiding place, behind the storeroom’s wooden wall slates. After the delivery, Al pleads with Gannon to release him from further gang participation because he fears detection. Gannon agrees to his release, but within several days, Al is lured into the warehouse where he is knifed to death by Bugsy with help from his cohort Red. When Bugsy’s application for Al’s job is rejected, Bugsy, concerned about the volatility of the firearm stash, advocates going ahead with their breakout with the weapons at hand, but Gannon refuses. In order to advance his plans, Gannon arranges to cast suspicion for Al’s murder on Rudy. Gannon orders Red to engage Rudy in a brawl,during which the murder weapon is slipped into Rudy’s pocket. After the fight, Rudy is brought before Starkey, but when Rudy insists that he knows nothing about the knife, Starkey orders him thrown into solitary confinement. Upon his release several days later, Rudy realizes that he stands little chance for an early parole and volunteers to help Gannon with his breakout plans. Gannon arranges to have the last gun barrel hidden in the warden’s car, which Rudy tends to weekly in the prison garage. A few days later, however, on a regular supply inspection, a guard accidentally discovers a gun barrel in a block of soap and presents it to Starkey and the warden. The warden orders all the prisoners confined to their work stations while the entire prison is searched for the guns. Realizing that Al’s murder must be connected with the illegal gun parts, Starkey inspects the storeroom. While Bugsy and his men anxiously watch, Starkey and the guards methodically go through several crates in the storeroom. Gannon leaves the office to join them and deliberately stands in front of the wooden wall to mislead the guards. Suspicious, Starkey orders the wall broken down and discovers a machine gun wrapped in newspaper hidden inside. After unwrapping the gun later in the warden’s office, however, the men discover the gun is over thirty years old. Doc later tells Bugs that he learned that the gun had been placed there years before as part of a failed escape plan. Rattled by the close call with the guards, Gannon decides to stage the escape for the next morning. Bugsy takes the morning guard hostage, and armed with the guard's gun, releases Gannon and the others, then locks up Starkey and the block guards. Gannon then frees the other inmates, who, with little encouragement, begin rioting. After Gannon receives assurances from the warden that the men will not be fired upon in the courtyard, he orders Bugsy to blow up the front gate at a specific time. Starkey tells Rudy that the gate is heavily fortified outside, and Rudy races to inform Gannon. Rudy locates Gannon just outside the plant nursery and is stunned to realize that he is abandoning the others to escape alone. When Rudy tells Gannon that the men will be slaughtered by the guards outside the gate, Gannon acknowledges that was part of his plan. Rudy tries to inform Bugsy and the others, but Gannon knifes him, then flees. Rudy staggers out to tell Doc to call off the escape, but Bugsy goes ahead and blows up the gate only to be killed in a hail of bullets from the guards outside. Rudy dies as Doc realizes the men never had a chance. In the present, the chief of detectives receives word from the hospital that Gannon has died.