Posing as husband and wife, Ruth Judlow and her brother-in-law Bill Judlow rent a house in the hills above the San Quentin Federal Penitentiary and prepare to free prisoner Arnie Judlow, Ruth’s husband and Bill’s twin brother. Arnie has devised a plan and sent instructions to Bill to assist in his escape. As they drive to the house with their supplies, Ruth admits that Arnie was intolerably possessive and violent. The ex-boxer is serving time for beating a man to death after the victim allegedly made sexual advances towards Ruth. After they arrive at the house, Ruth tells Bill about the prison schedule: At 4:00 p.m. the prisoners leave the textile mill and make their way from the industrial area back to their cells. Once all the prisoners are accounted for, the industrial wall is left unguarded. An uninterrupted beam of light from the prison’s main beacon signifies that all prisoners are in their cells. If a prisoner is missing, the light flashes and the guards patrol the industrial area twenty-four hours a day until the prisoner is found. The day after Ruth and Bill’s arrival, they hear a radio broadcast reporting that a guard attacked early in the week by an unidentified prisoner is still unconscious and unable to identify his assailant. Ruth and Bill know Arnie committed the crime and that if convicted of the assault, Arnie’s life sentence will be converted to a death sentence. Later that night, neighbor Henry Nova, a guard at the prison, recognizes Ruth as a regular visitor. Bill introduces Ruth as his wife and explains that his brother is in San Quentin. After leaving Henry, Ruth and Bill decide to continue as planned even though Henry can now identify them. In the middle of the night, Ruth drives Bill to the industrial area wall. Throwing a hook onto the top of the wall, Bill pulls himself and his equipment safely into the prison and then hides in the crates outside the mill. The next day at 4:00 p.m. Arnie leaves the mill and surreptitiously switches places with his brother. As Arnie hides in the crate, Bill returns to the cellblock with the other prisoners. In the mess hall that evening, Bill lights up a cigarette, unaware that it is against regulations. After Bill’s violation is written up, a suspicious Henry takes him outside and tells him he met his brother and his brother’s wife. Late that night, Arnie climbs out of the crates, digs a rectangular hole near the wall, covers it with boards and grass sod and returns to the crates. The next morning, outside the mill, as Bill and Arnie switch places, Arnie viciously jokes that he could have escaped and left his brother in prison. That night, Bill pulls himself over the wall where Ruth is waiting in a car. After Bill carves a gun from a bar of soap the next morning, he reminisces with Ruth about his childhood. Despite his dreams of becoming an architect, his parents had only enough money to send Arnie, the brighter of the two, to college. Ruth tells him that Arnie dropped out of college to box, which lead to him using his fists as “deadly weapons” against other men. She regrets marrying him, explaining that his possessiveness caused him to kill, but Bill insists the crime was not her fault. That evening, Arnie sets a fire in the trashcan outside the mill after work to divert attention while he slips into the hole. When the beacon begins blinking after the count comes up short that night, Ruth drops Bill, now dressed in prison uniform on a road near the prison. After planting prison-issue cigarettes at an intersection, Bill pulls a gun on an unsuspecting driver, orders him out of the car and then drives away. When the driver reports the crime, the police find the San Quentin tobacco as well as Arnie’s identity card and the hand-carved gun, the signature weapon of an escaped convict. That night, the beacon shines without blinking when the warden concludes that Arnie has escaped the prison grounds. When Ruth and Bill return to the house, they are met by Henry, who accuses Bill of switching places with his brother and deduces that Arnie will now try to escape. Bill offers Henry $5,000 to keep quiet, $2,500 up front and the rest later. Henry agrees but holds Ruth hostage while Bill gets the remaining money. Bill then goes to the prison to free his brother, but when they reach the other side of the wall, Henry is waiting with a gun. After Arnie beats Henry unconscious, Bill takes the money and gun and insists they let the guard go, explaining that Henry will not reveal their escape for fear of being jailed for withholding information. When they arrive at the house, Bill gives Arnie his car, the keys to an apartment and $2,500 to start a new life. When Ruth is reluctant to join Arnie, he assumes she and Bill have been having an affair and psychotically lashes out at both of them. When he finally calms down and apologizes, Arnie admits that Ruth cannot live with him on the run and leaves. Later that night, the warden sends a guard to escort Ruth and Bill to the prison where he tells them an anonymous caller described the crime in detail and stated that Bill assisted in Arnie’s escape. The persuasive warden offers to clear Bill and Ruth of any charges if they tell him Arnie’s location, warning that if the mentally unstable man is not apprehended he will soon kill again. In private, Ruth and Bill realize that Henry could not have provided the warden with the details of the crime and conclude that the caller was Arnie, who, cognizant that he cannot control himself, wants to be caught. Bill writes down Arnie’s apartment address and hands it to the warden, who tells Bill that he has saved his brother from the gas chamber. The warden places a call, ordering that Arnie be taken alive.