Alcala Prison is located in the San Francisco Bay, USA. It was the prison with the highest defense density at that time, and the most heinous criminals were held there. This prison is known as the most difficult prison to escape from in history. Frank (Clint Eastwood) was imprisoned in Alcala Prison for the crime of bank robbery. Here, he met Limutus (Frank Ronzio), who was good at taming mice, and the genius painter "Doctor" (Roberts Blossom), who had excellent artistic cells. Because of his good performance and high IQ, Frank was sent by the warden to work in the prison library, where he met the prisoner leader Inglis (Paul Benjamin). The warden's arbitrary and brutal rule gradually made the prisoners resentful. After a series of radical events, Frank decided to lead everyone to escape from prison.