After John Dortmunder (Redford) is released from his latest stint in prison, he is approached by his brother-in-law, Andy Kelp (Segal), about another job. Dr. Amusa (Gunn) seeks a valuable gem in the Brooklyn Museum that is of great significance to his people in his country in Africa, stolen during colonial times and then re-stolen by various African nations.
Dortmunder and Kelp are joined by driver Stan Murch (Leibman) and explosives expert Allan Greenberg (Sand), concocting an elaborate plan to steal the gem. Although the scheme (and each subsequent one) is carefully planned—and keeps increasing in cost—something always goes awry, and the quartet has to steal the diamond again and again.
First off, the diamond is swallowed by Greenberg when he alone gets caught by the museum guards during the initial heist. Dortmunder, Kelp, and Murch, at the urging of Greenberg's rotund father Abe (Zero Mostel), a lawyer, help Greenberg escape from state prison, but they then find he does not have the diamond. After Greenberg tells his partners he hid the rock in the police station (after bodily evacuating it), the quartet break into the precinct jail by helicopter, but the rock is not where Greenberg hid it. Greenberg discloses that his father Abe was the only other person who knew where it was.
It isn't until Murch, disguised as the grunting muscle man "Chicken," threatens Abe with being thrown down an elevator shaft, that Abe gives up the location of the diamond—his safe deposit box, and he also gives up the key to it. However, Dortmunder cannot access the box because of bank vault security, and the gang leaves Abe in Dr. Amusa's office while they come up with a plan.
With the help of a hypnotist by the name of Miasmo, Dortmunder sets up his own safe deposit box to get access to the vault and then plans to invoke the predetermined hypnotic trigger word "Afghanistan bananastan" to the vault guard. He would then be able to gain access to Abe's safe deposit box and retrieve the gem just after the bank opens in the morning.
While Dortmunder is waiting for the bank to open, the rest of the group meets with Dr. Amusa at his request. Dr. Amusa fires them for incompetence, and reveals that Abe Greenberg has made his own deal to sell him the gem, which will leave Dortmunder's gang with nothing.
Dortmunder finally retrieves the gem while Dr. Amusa and Abe are driving to the bank by limousine. He exits the bank and walks away just before they arrive. Dortmunder climbs into Kelp's car where the others are waiting, and a rousing cheer erupts as they drive off.
The screenplay was by William Goldman, who called Westlake's book "wonderful, very funny... I was a Westlake fan."
As Dortmunder's gang flies through Manhattan to break into the police station, their helicopter flies by the World Trade Center. The south tower is clearly seen as still being under construction in several shots.
Frank P. Keller and Fred W. Berger were nominated for the best film editing Academy Award. William Goldman was nominated for the Edgar Allan Poe Award for his screenplay.