Frank Leone, a skilled mechanic and football player in Hoboken, New Jersey, is a model prisoner nearing the end of his sentence in Norwood, a low security prison. He occasionally spends time outside prison in his garage fixing cars, playing football and spending time with his girlfriend Melissa.
One night, while sleeping in his cell, guards arrive and forcibly take Leone to maximum security Gateway Prison run by Warden Drumgoole. Drumgoole explains to him that he will serve hard time because he escaped once from Treadmore and did so on Drumgoole's watch. He escaped because his mentor and friend was dying; Leone was refused even one hour to see him so Leone escaped to visit him and went to the press about the warden's treatment of his prisoners, resulting in Drumgoole's transfer to Gateway and Leone serving five additional years in minimum security before his transfer.
Leone is mistreated by the guards and the boss of the inmates, Chink Weber, who is Drumgoole's muscle appendage. There he befriends fellow prisoners Dallas, Eclipse, and First-Base, and shows them his concept of freedom that has helped Leone go on: that the body may be confined to one place, but "your mind can be wherever you want it". The foursome refurbish a Ford Mustang, which Eclipse nicknames "Maybelline". Leone explains to Eclipse that he was sent to prison for taking the law into his own hands when he avenges an attack on his father. After Leone reluctantly allows First-Base to start the car he refuses to turn it off and drives the Mustang out of the garage. Drumgoole makes them watch as inmates destroy the car and Leone is subsequently sent to solitary confinement for six weeks. There Leone is tortured by the guards who wake him at random points in the middle of the night, forcing him to face a security camera and dictate his name and prisoner number. However, the guards' captain, Meissner, and one other guard become so disgusted with the crude sadism of the warden and his toadies that Meissner orders it to stop and releases Leone from confinement.
Aiming to force Leone to snap and compromise his position, the warden has Chink Weber kill First-Base in the gym. Enraged, Leone fights and defeats Chink, but spares his life, knowing a dead inmate by his hands is precisely what Drumgoole wants. However, one of Chink's goons stabs Leone from behind with a shank. As Leone recovers in the prison infirmary, a stranger tells him that he has been hired to rape and murder Melissa, then leaves. That night, Leone attempts his escape with Dallas to try and save Melissa, however Dallas lures him to a dead end where they are captured by Drumgoole and his guards. It is revealed that Drumgoole had staged the fake rape attempt to force Leone to escape and receive a mandatory 10-year sentence, and that he had enlisted Dallas to set Leone up in exchange for an early release. Drumgoole however denies his part of the deal with Dallas and leaves the two inmates to be abused by the guards. A struggle between Leone and the guards ensues and Dallas, knowing the other prisoners would kill him after setting up Leone, apologizes to Leone and electrocutes himself and officer Manly to help Leone escape.
Enraged, Leone breaks into Drumgoole's office, takes him to the execution chamber and straps him to the electric chair. He activates the generator and secures his hand to the switch. The prison guards break into the execution chamber and point their weapons at Leone, but don't shoot because of the situation. Under threat of being executed, the warden finally confesses to his plot to increase Leone's jail time. Leone pulls the switch anyway but nothing happens; he then reveals he took one of the fuses out before to trick the warden into confessing. Captain Meissner and his men cuff Frank, but they also take Drumgoole into custody for the legal confession.
A judicial inquiry is made into the matter about the warden's corrupt behavior and Leone serves only the jail time required of him in the first place. A few weeks later Frank leaves prison to the cheers of his fellow inmates and meets up with Eclipse one last time. He wishes Captain Meissner farewell, exits Gateway and embraces the waiting Melissa.Sylvester Stallone as Frank Leone
Donald Sutherland as Warden Drumgoole
John Amos as Captain Meissner
Sonny Landham as Chink Weber
Tom Sizemore as Dallas
Frank McRae as Eclipse
Darlanne Fluegel as Melissa
William Allen Young as Braden
Larry Romano as First Base
Jordan Lund as Manly
John Lilla as Wiley
Dean Duval as Ernie
Jerry Strivelli as Louie Munafo
David Anthony Marshall as Mastrone
Kurek Ashley as Chink's Gang Member
Michael Petroni as Chink's Gang Member
Danny Trejo as Chink's Gang Member
Frank Pesce as Johnson
Lock Up grossed $22.1 million on a budget of $24 million.
Rotten Tomatoes, a review aggregator, reports that 17% of twelve surveyed critics gave the film a positive review; the average rating is 4.2/10. Kevin Thomas of the Los Angeles Times wrote that Stallone "defies credibility to the point of inviting unintended laughter." Hal Hinson of The Washington Post wrote, "Lock Up bears the unmistakable mark of a vanity production".
The film was nominated for three Razzie Awards including Worst Picture, Worst Actor for Sylvester Stallone and Worst Supporting Actor for Donald Sutherland, but failed to win any of those categories.