On the night of 18 January 1960, convict Frank Morris (Clint Eastwood) enters the federal penitentiary at Alcatraz, an island fortress situated in San Francisco bay, California. After being strip-searched and given a dark blue prison uniform, Frank is escorted naked to his cell where one guard taunts him by saying "Welcome to Alcatraz". Soon after arriving, Frank is summoned to the office of the warden (Patrick McGoohan), who warns him that Alcatraz is unique within the U.S. prison system for its exceedingly high level of security. According to his file, Frank has an exceptional I.Q. and has absconded from other facilities, but the warden informs him that no inmate has ever successfully escaped from Alcatraz. During the conversation, the warden fails to notice that Frank has stolen one of the nail clippers on the desk.Over the next several days, Frank makes acquaintances with a few inmates. Among them are "Litmus" (Frank Ronzio), a lifer who keeps a pet mouse, "English" (Paul Benjamin), an African American man serving two consecutive 99 year sentences for a double murder and the prison librarian, and Chester "Doc" Dalton (Roberts Blossom), an elderly man who paints portraits and once grew chrysanthemums at Alcatraz. However, Frank also encounters a bully named Wolf (Bruce M. Fischer), who harasses him in the showers and later attacks him in the prison yard with a shank. The guards break up the fight, and both men are sent to solitary confinement cells in D block, despite Frank merely acting in self-defense.Two weeks later, after being released from the brutal, isolated conditions, Frank settles into the routine of Alcatraz and a job in the carpentry shop and running books for English from the library. English commands deep respect in the prison yard, sitting high above the other inmates. When Frank approaches, English's friends try to stop him but English unexpectedly permits him to sit with him.Frank also befriends a new inmate in the adjoining cell, Charley Butts (Larry Hankin), in on a grand-theft auto charge that became a federal rap when he crossed state lines with the car. When the warden enters Doc's cell unannounced, he sees that doc has painted a very detailed portrait of the warden. At first the warden appears flattered but later he cruelly orders Doc's painting privileges to be eliminated without explanation. Devastated, Doc intentionally chops off three of his fingers in the carpentry shop with a hatchet. Frank and the other inmates soon learn that the warden saw the portrait of himself among Doc's paintings and was responsible for ordering the sadistic penalty.In his cell one day, Frank sees a roach climb into the ventilator grill underneath the sink. Using the nail file he stole from the warden, he chips off a piece of the concrete around the opening. Along with brothers John and Clarence Anglin (Fred Ward and Jack Thibeau), who are Frank's friends from another prison sentence, and Charley, Frank plans an escape. Because the moist air and the salt water surrounding Alcatraz have softened the concrete foundation and corroded the metal over the last several decades, Frank thinks they can dig an opening around their ventilators large enough to crawl through and access the shaft leading to the roof.To mislead the guards during head counts, Frank suggests making papier-mâché dummy heads, using flesh-colored paint and hair from the barbershop. Since John works in the clothing shop, he can secure raincoats that can be combined with contact cement to make a raft and life preservers. Charley takes up painting to procure the colors needed to create the dummy heads and create cardboard coverings for the cell vents. Over the next several nights, as Charley keeps watch, Frank continues to scoop out his opening with the nail file and uses an accordion to disguise the hole. He later sneaks a spoon from the cafeteria and welds it to the file to provide a better grip for digging, a trick he learns from English using filings from a dime he gets from Litmus (who happily demands Frank's desserts at chow time).Using a wedge he sneaks from the carpentry shop, he is able to kick out the metal grill and takes his first peak into the shaft. Charley and the Anglins start to chisel their openings, while Frank creates his dummy head. When the decoy is finished, Frank lays the fake head on the cot as if he is sleeping and takes his first trip into the passageway. Initially the guard is fooled by the dummy, but later becomes suspicious when he notices that Frank has not moved. However, Frank miraculously returns to the cot before the decoy is exposed.The following day, Frank learns that the Anglin brothers are making progress on their openings and heads, but Charley is behind schedule. During the next trip into the passageway, John brings the raincoats and begins preparing the flotation devices. Meanwhile, Frank investigates the iron rods and hood blocking the exit to the roof and devises a makeshift drill from a small fan to remove their structural support. With the passageway clear and the rafts almost ready, Frank schedules the escape for Tuesday night, June 11, 1962.A few days later on the day of the planned escape, during mealtime, Frank places a chrysanthemum at the table in honor of Doc, but the warden stops by and crushes it, reminding Frank of regulations. Litmus is enraged, but as he reaches out to attack the warden, he suffers a fatal heart attack. Later, the warden requests a shakedown of Frank's cell and finds nothing unusual. However, when he hears that Frank and Charley stay up late conversing, he issues orders for the two inmates to be relocated to different cells as soon as possible. Meanwhile, Wolf has been released from solitary confinement and prepares to stab Frank with a knife, but English is able to intercept him.That night, Frank, John, Clarence and Charley plan to meet in the passageway and escape. However, Charley panics and fails to rendezvous with them. Carrying the flotation gear, Frank and the brothers access the roof and avoid the searchlights. From there, they scramble down the side of the building into the prison yard, climb over a barbed-wire fence and make their way to the shore of the island where they inflate the raft. The three men enter the water and begin kicking.The next day at dawn, a guard orders Frank to get up and prepare for relocation to a new cell. When there is no response, the guard slaps the head and it rolls on the floor. The alarm sounds and English smiles, knowing that Frank has escaped. As police search the area, there is no evidence to indicate whether or not the three inmates survived, but the warden does not want to blemish his perfect record and insists they drowned. On a rock by the shore, he finds a chrysanthemum and throws it in the water. The fates of Frank, Clarence and John are left unknown even after an extensive investigation by the FBI.