Aging criminal mastermind John Egan Crahan Denton decides to rob St. Louis' Southwest Bank, then use the loot to retire to Mexico with his longtime minion Willy James Dukas. Their accomplice, Gino David Clarke, is an ex-convict terrified of returning to prison, and wants money to pay for his defense in an upcoming criminal trial. The gang needs a driver for the getaway car. Gino recruits his sister Ann's old boyfriend, George Fowler Steve McQueen, a former collegiate football star with no criminal history. Egan takes a liking to George, much to Willy's displeasure.
The gang meticulously gathers information on the bank to help Egan draw up his plans. In order to pay for a hotel room, Gino convinces George to borrow money from Ann Molly McCarthy. The transaction is the first meeting between the young couple since an unspecified action by George got them both expelled from college. George and Gino keep the incident hidden from Egan, who refuses to have women even tangentially involved in his work; he drunkenly confesses to George that his misogyny comes from experiences with his abusive mother, whom he eventually murdered by pushing her down a flight of stairs.
Ann sees George and Gino watching the bank, and deduces that they are planning a robbery. She confronts George, who claims that he will use the money to return to school and straighten out his life. Ann still feels too much affection for George to notify the police, but she goes to the Southwest Bank at night and writes a warning about the impending robbery on its window. Willy saw George and Ann together, enabling Egan to trace the source of the message. The gang invades Ann's apartment, with Egan demanding that she leave town until the heist has been completed. Both Gino and George fail to stand up for Ann, letting Egan and Willy carry her away. As the three leave the apartment building, Egan has a flashback to his mother's murder and throws Ann to her death off the fire escape.
Feeling personally betrayed by George, Egan orders him to participate directly in the robbery while Willy drives the car. They neglected to bring a police-frequency scanner and are unaware that the bank had relocated a switchboard from the lobby, elements that foil key aspects of their plan. An employee triggers the silent alarm, and police swarm the bank exterior. Willy flees in the car, leaving the other robbers stranded. As the gang try to work out their next move, Egan inadvertently reveals that he killed Ann. He forces his way out of the bank, using a teller as a human shield, but is shot down by the police. Gino dies by suicide in the basement vault.
George, wounded by a shot in the leg, takes a young woman hostage. Her husband tries to intervene, and the woman warns him that George is a vicious criminal who will show them no mercy. Already shaken by Ann's death, George breaks down and begins protesting that he is not really a criminal. He releases the woman and lets himself be captured by the police. Driven away in a paddy wagon, he sees the world receding behind metal bars.
