First-time crook Sonny Wortzik (Al Pacino), his friend Salvatore "Sal" Naturale (John Cazale), and Stevie (Gary Springer) attempt to rob the First Brooklyn Savings Bank. The plan immediately goes awry when Stevie loses his nerve shortly after Sal pulls out his gun, and Sonny is forced to let him flee the scene. In the vault, Sonny discovers that he and Sal have arrived after the daily cash pickup, and only $1,100 in cash remains in the bank.
To compensate, Sonny takes a number of traveler's cheques, but his attempt to prevent the cheques from being traced by burning the bank's register in a trash can causes smoke to billow out the side of the building, alerting the business across the street to suspicious activities. Within minutes, the building is surrounded by the police. Unsure of what to do, the two robbers camp out in the bank, holding all the workers hostage.
Police Detective Sergeant Eugene Moretti (Charles Durning) calls the bank to tell Sonny that the police have arrived. Sonny warns that he and Sal have hostages and will kill them if anyone tries coming into the bank. Sal tells Sonny that he is ready to kill the hostages if necessary. Detective Moretti acts as hostage negotiator, while FBI Agent Sheldon (James Broderick) monitors his actions.
Howard Calvin (John Marriott), the security guard, has an asthma attack, so Sonny releases him when Moretti asks for a hostage as a sign of good faith. Moretti convinces Sonny to step outside the bank to see how aggressive the police forces are. Using head teller Sylvia "The Mouth" (Penelope Allen) as a shield, Sonny exits the bank and begins a dialogue with Moretti that culminates in his shouting "Attica! Attica!" (invoking the recent Attica Prison riot), and the civilian crowd starts cheering for Sonny.
After realizing they cannot make a simple getaway, Sonny demands that a helicopter be landed on the roof to fly him and Sal out of the country. When they are informed that the asphalt roof of the bank will not support a helicopter, Sonny demands that a vehicle drive him and Sal to an airport so that they can board a jet. He also demands pizzas for the hostages (which are delivered to the scene) and that his wife be brought to the bank. When Sonny's wife, Leon Shermer (Chris Sarandon), a pre-operative transsexual, arrives, she reveals to the crowd and officials one of Sonny's reasons for robbing the bank is to pay for Leon's sex reassignment surgery, and that Sonny also has an estranged divorced wife, Angie (Susan Peretz), and children.
As night sets in, the lights in the bank all shut off. Sonny goes outside again and discovers that Agent Sheldon has taken command of the scene. He refuses to give Sonny any more favors, but when the bank manager, Mulvaney (Sully Boyar), goes into a diabetic shock, Agent Sheldon lets a doctor (Philip Charles MacKenzie) through. While the doctor is inside the bank, Sheldon convinces Leon to talk to Sonny on the phone.
The two have a lengthy conversation that reveals Leon had attempted suicide to "get away from" Sonny. She had been hospitalized at the psychiatric ward of Bellevue Hospital until the police brought her to the scene. Leon turns down Sonny's offer to join him and Sal to wherever they take the plane. Sonny tells police listening to the phone call that Leon had nothing to do with the robbery attempt.
After the phone call, the doctor asks Sonny to let Mulvaney leave and Sonny agrees. Mulvaney refuses, instead insisting that he remain with his employees. The FBI calls Sonny out of the bank again. They have brought his mother to the scene. She unsuccessfully tries persuading him to give himself up, and Agent Sheldon signals that a limousine will arrive in 10 minutes to take them to a waiting jet. Once back inside the bank, Sonny writes out his will, leaving money from his life insurance to Leon for her sex change and to Angie.
When the limousine arrives, Sonny checks it for any hidden weapons or booby traps. When he decides the car is satisfactory, he settles on Agent Murphy (Lance Henriksen) to drive Sonny, Sal, and the remaining hostages to Kennedy Airport. Per Sonny's earlier agreement, an additional hostage, Edna (Estelle Omens) is released, and the remaining hostages get into the limousine with Sonny and Sal. Sonny sits in the front next to Murphy while Sal sits behind them. Murphy repeatedly asks Sal to point his gun at the roof so Sal won't accidentally shoot him.
As they wait on the airport tarmac for the plane to taxi into position, he again reminds Sal to aim his gun up so he does not fire by accident. Sal does so, and Agent Sheldon forces Sonny's weapon onto the dashboard, creating a distraction which allows Murphy to pull a revolver hidden in his armrest and shoot Sal in the head. Sonny is immediately arrested and the hostages are all escorted to the terminal. The film ends with Sonny watching Sal's body being taken from the car on a stretcher. Subtitles reveal that Sonny was sentenced to 20 years in prison, Angie and her children subsisted on welfare, and Leon had her sex reassignment surgery.
Output: Why does Sonny release the security guard?