On the night of January 18, 1960, Frank Morris (Clint Eastwood) arrives at the notorious Alcatraz Federal Penitentiary, an isolated fortress nestled in San Francisco Bay, California. After undergoing a strip search and donning a somber blue prison uniform, Frank is led, stark naked, to his cell, where a guard mockingly greets him with, > "Welcome to Alcatraz." Shortly after, Frank meets with the warden (Patrick McGoohan), who emphasizes that Alcatraz is unlike any other U.S. prison due to its unparalleled security. Though his file reveals that Frank possesses a remarkable I.Q. and has previously escaped from other facilities, the warden grimly informs him that no inmate has ever broken free from Alcatraz. Unbeknownst to the warden, Frank discreetly pilfers a nail clipper from his desk.
In the days that follow, Frank forms connections with fellow inmates, including "Litmus" (Frank Ronzio), a lifer with a pet mouse; "English" (Paul Benjamin), an African American man serving two consecutive 99-year sentences for double murder and the prison's librarian; and Chester "Doc" Dalton (Roberts Blossom), an elderly artist who once nurtured chrysanthemums at Alcatraz. However, he also encounters a hostile inmate named Wolf (Bruce M. Fischer), who harasses him in the showers and later assaults him in the yard, leading them both to solitary confinement in D block, despite Frank acting in self-defense.
Weeks later, after adapting to the harsh routines of Alcatraz and settling into a carpentry job while running books for English, Frank earns respect among the inmates, particularly from English, who reigns with authority in the yard. Frank befriends a new inmate in the adjoining cell, Charley Butts (Larry Hankin), who’s incarcerated for grand theft auto that escalated into a federal offense due to crossing state lines. One day, the warden unexpectedly inspects Doc’s cell, discovering an impressively detailed portrait of himself. Initially pleased, he cruelly revokes Doc's painting privileges, leading the distraught inmate to mutilate himself in despair.
While in his cell, Frank notices a roach crawling into the vent beneath his sink. Utilizing the stolen nail file, he begins to chip away at the concrete surrounding the vent. Together with brothers John and Clarence Anglin (Fred Ward and Jack Thibeau), who are familiar faces from Frank's previous incarceration, as well as Charley, he devises a cunning escape plan. Observing how the salty air and moisture have weakened the concrete over decades, Frank believes they can create an opening large enough to maneuver through and access the shafts leading to the roof.
To distract the guards during head counts, Frank cleverly proposes the creation of papier-mâché dummy heads, complete with realistic skin tones and hair obtained from the prison barbershop. Since John is employed in the clothing shop, he can easily obtain raincoats to craft into flotation devices. Charley picks up painting to reproduce the colors required for the dummies and to make cell vent covers. As the nights pass, with Charley on watch, Frank diligently works on the escape route using the nail file, disguising his efforts with an accordion. He then manages to steal a spoon from the cafeteria and rigs it to the file for better leverage, learning this trick from English through small favors exchanged for desserts shared during mealtime.
With a sneaked wedge from the carpentry shop, he successfully removes the metal grill, granting him his first glimpse into the shaft. While the Anglin brothers labor to chisel their openings, Frank meticulously creates his dummy head. After finishing it, he places the decoy on his cot to give the illusion of sleeping, successfully fooling the guard—at least for a while. However, suspicion soon arises when the guard realizes Frank has not moved. Nevertheless, Frank cleverly manages to return to his cot just in time.
The following day, Frank discovers the Anglin brothers are making headway with their dummies, but Charley is lagging behind. During a subsequent excursion into the passage, John brings the assembled raincoats, and preparations for their flotation devices commence. Moreover, Frank assesses the iron rods and hood blocking their exit to the roof, devising a makeshift drill using a fan for extraction. With the passages ready and flotation gear nearly complete, they set their escape for the evening of June 11, 1962.
On the day of the escape, during mealtime, Frank places a chrysanthemum on the table to honor Doc's plight. Disregarding the sentiment, the warden crushes the flower and reminds Frank of the rules. Litmus, enraged by the act, attempts to retaliate but sadly suffers a fatal heart attack instead. Later, the warden orders a shakedown of Frank’s cell, but nothing amiss is uncovered. However, after hearing that Frank and Charley have been staying up late talking, the warden insists they be relocated without delay. Meanwhile, the vindictive Wolf is released from solitary and plans to attack Frank, though English intervenes just in time.
That fateful night, when Frank, John, Clarence, and Charley gather in the passage for their escape, Charley panics and fails to join them. With the flotation gear in hand, Frank and the Anglin brothers access the roof, skillfully evading searchlights. They then descend into the prison yard, climb over a barbed-wire fence, and reach the island shore where they begin inflating their raft. Together, they plunge into the water, kicking furiously to swim.
The next day, a guard calls for Frank to rise and prepare for a cell relocation. When silence follows, he approaches the cot to find the decoy head rolling on the floor. As alarms blare, English shares a knowing smile—Frank has indeed escaped. As law enforcement canvasses the area, they find no traces confirming the inmates' survival. The warden, eager to safeguard his impeccable record, asserts that they must have drowned. In a moment of sorrow, he spots the chrysanthemum left on the rocks and tosses it into the water, leaving the fates of Frank, Clarence, and John shrouded in mystery, even after an exhaustive FBI investigation.
