The inmates of Westgate Penitentiary endure their brutal existence, terrorised by sadistic guard Captain Munsey. Even Munsey's boss the weak-willed warden is afraid of him. Only inmate Joe Collins and the sympathetic but powerless prison doctor remain unafraid.
Joe's cellmates kill Munsey's informant. Munsey tries to get Joe's cellmate Tom Lister to inform on Joe but Tom refuses. Munsey lies that Tom's wife Cora wishes a divorce and Tom hangs himself. Blaming the cellmates for the suicide, Munsey assigns them to work in the most feared place--the drains. Joe and influential inmate Gallagher plan an escape through the drains.
Munsey tortures one of Gallagher's men, Miller, even though he knows the details of Joe's escape plan. The doctor visits Joe in the drains and begs him not to escape because Munsey knows, but Joe goes ahead with the plan. Joe finds out the identity of Munsey's new informant and kills him. Most of Joe's fellow cellmates are killed by the guards and Joe himself is shot but he makes his way to the top of the prison watchtower. As Gallagher leads the escape attempt, the inmates riot. Munsey opens fire with a machine gun on the inmates from the watchtower. Gallagher dies after being shot. Joe and Munsey fight and Joe throws Munsey off the watchtower, killing him. The riot is quelled and none of the prisoners manage to escape. Later the doctor announces Joe's death.