Paul “Wrecking” Crewe is a former star pro football quarterback who walks out on his wealthy girlfriend Melissa in Palm Beach, Florida. He takes her Maserati-engined Citroën SM without permission and leads police on a car chase. Crewe is eventually caught and sentenced to 18 months in Citrus State Prison.
The convicts disrespect Crewe because he was dismissed from the NFL for point shaving. The warden, Rudolph Hazen, is a football fanatic who manages a semi-pro team of prison guards. He wants Crewe to help coach the team and clinch a championship. Responding to pressure from the head guard and coach, Captain Wilhelm Knauer, a reluctant Crewe eventually agrees to play in an exhibition game. Crewe forms a prison team that includes Samson, a former professional weightlifter, and Connie Shokner, a serial killer and martial arts expert.
With the help of the clever Caretaker, former professional player Nate Scarboro and the first black inmate willing to play, “Granny” Granville, plus long-term prisoner Pop—and with an assist from the warden’s amorous secretary, Miss Toot—Crewe molds a team nicknamed the “Mean Machine”. He agrees to play quarterback himself. After witnessing “Granny” being harassed by some of the prison guards without breaking, the black inmates decide to volunteer their services and join the team. Unger, one of the prison trustees, persistently asks Crewe if he can replace Caretaker as manager of the team, which Crewe refuses to do. In retaliation, Unger attempts to kill Crewe by fashioning a home-made bomb from a light bulb filled with a combustible fluid, designed to detonate inside Crewe’s cell when he turns on the light. However, Caretaker is killed instead, when he enters Crewe’s cell to retrieve some papers and Unger closes the cell door, locking him in and preventing rescue. Crewe’s teammates are given a stern lecture from Hazen about the consequences of any attempted escape after the game. Afterward, Crewe re-energizes the team with a surprise – presenting them with professional uniforms (stolen from the guards by Caretaker before he was killed). They charge onto the field, to the shock of the guards and Hazen, in their new uniforms.
The “Mean Machine” starts out surprisingly well, and at halftime the game is close, with the guards leading, 15-13. Hazen threatens Crewe as an accessory to Caretaker’s murder unless Crewe loses the game to the guards by at least 21 points. Crewe reluctantly agrees, but only if Hazen promises not to hurt the other prisoners; Hazen agrees, but in bad faith, and tells Knauer to have his team “inflict as much physical punishment on the prisoners as humanly possible” as soon as they are ahead by 21 points. Crewe makes deliberate mistakes, putting the “Mean Machine” down by more than three touchdowns, 35-13, then takes himself out of the game. The guards gladly injure several of the prisoners, and Crewe’s teammates feel betrayed.
Depressed, Crewe goes back into the game, but the prisoners refuse to co-operate with him until he convinces them of his change of heart. The “Mean Machine” gets back into the game, trailing 35-30, one of their touchdowns scored by Nate despite his bad knee, and he is immediately cut down and crippled by guard Bogdanski. As he is wheeled off the field, Nate tells Crewe to “screw Hazen” and win the game. Crewe scores the winning touchdown with no time left and the “Mean Machine” wins, 36-35.
As the prisoners celebrate, Crewe walks away across the field towards the departing crowds. Hazen furiously repeatedly orders Knauer to shoot him because “he’s trying to escape.” But he hesitates due to his new found respect for Crewe who is actually retrieving a football. Disgusted at what he almost did, Knauer hands the rifle back to Hazen saying “game ball”. Crewe reaches Hazen and hands him the ball, saying “Stick THAT in your trophy case.”