Roy Tucker, Gene Hackman, is serving time in San Quentin for the murder of his wife's first husband. He is introduced to Marvin Tagge, Richard Widmark, by Warden Ditcher, Ken Swofford. Through a series of interviews with Tagge and an associate named Ross Pine, Edward Albert Tucker learns that the two men represent a shadowy organization, and Tagge presents him with an offer: in exchange for helping him escape and start a new life, Tucker must work for the organization for a few weeks. Tucker is initially wary of the offer and confides in his cellmate Oscar Spiventa, Mickey Rooney, who warns him that he is being groomed as an expendable hitman by "them," a nameless cabal that runs the country from the shadows. Tucker dismisses Spiventa as paranoid.
But the lure of reuniting with his wife Ellie, Candice Bergen, proves too strong, and he agrees. Spiventa declines Tucker's invitation to join the escape, but changes his mind at the last moment. With arrangements made for the prison staff to turn a blind eye, the two convicts are driven out the main gate in a bread delivery truck to a rendezvous point underneath the Golden Gate Bridge. The two organization agents there immediately kill Spiventa and take Tucker to the Hyatt Regency San Francisco, where he meets General Tom Reser, Eli Wallach, a confederate of Tagge and Pine. Tucker is allowed to enjoy himself in the city before he is taken to his wife. He learns from news broadcasts that Spiventa's body was found and authorities believe he committed the murder and escaped to Canada. He also surreptitiously meets with his lawyer, Arnold Schnaible, Ted Gehring, only for Schnaible to turn up dead soon afterward.
Tucker is flown to Puntarenas, Costa Rica, where he is given a bank account with $200,000 and a house to share with Ellie, who was led to believe that Tucker had been released from prison pending a new trial. After a few idyllic days, Tagge, Pine and Reser return the couple to Los Angeles. Tucker is prepared for his task by being directed by Reser to shoot tin cans with a rifle while riding a helicopter with military livery, after which the helicopter does a flyby of a rural estate. He realizes that he is expected to assassinate a politician and tries to back out. The organization retaliates by kidnapping Ellie and Tucker submits.
The next morning, Tucker shoots the target as planned, though the getaway is marred by the target's security detail damaging the helicopter and mortally wounding the pilot, forcing it to land and be abandoned in the mountains, where it is blown up along with the pilot's body. At a hideout, Tucker takes Pine hostage, demanding a plane and the return of his wife. Tagge complies. At Burbank Airport, Tucker tells Tagge that he deliberately fired short, so he knows someone else killed the target. Tagge reveals that two other shooters were in place, including Spiventa, who is not only still alive but was recruited in 1961; Tucker himself has been manipulated by the organization for over a decade. Tucker asks, "Is it over?", to which Tagge replies that it all depends on the man who gave the original order for the assassination: "If he panics, then the dominoes start to fall."
Aboard the plane with Ellie, Tucker spots someone planting a toolbox in the back of Tagge's car. Unable to get the pilot to abort takeoff, Tucker watches helplessly as Tagge is blown up with his car. The couple return to Costa Rica where Tucker sees his fragile new life dismantled as quickly as it was assembled: his false passport destroyed, his bank account emptied and Ellie run into and killed by a passing truck.
Spiventa and Pine arrive; Tucker shoots them both and dumps their bodies in the ocean. The film closes with a resolute Tucker vowing not to give up as he walks down a beach with a rifle, unaware he is in the crosshairs of yet another assassin.
