Along the U.S.–Mexico border, a time bomb placed inside a vehicle explodes, killing Rudy Linnekar and his stripper girlfriend Zita. On his honeymoon with his American wife Susie, Mexican special prosecutor Miguel Vargas takes an interest in the investigation. Local authorities arrive on the scene, followed by 30-year police captain Hank Quinlan and his longtime assistant, Pete Menzies. Quinlan and Menzies implicate Sanchez, a young Mexican secretly married to the victim's daughter, Marcia, as the prime suspect. During the interrogation at Sanchez's apartment, Menzies locates two sticks of dynamite in the same shoebox that Vargas had found empty only a few minutes earlier. Vargas accuses Quinlan of planting evidence, and begins to suspect that he may have been doing so for years to help win convictions. Quinlan dismisses Vargas's claim, saying he is just biased in favor of fellow Mexicans.
The stress of these accusations, along with pressure from "Uncle" Joe Grandi, the brother of a man Vargas has been investigating, to strike a deal to discredit Vargas, causes Quinlan—who has been sober for 12 years—to relapse. With assistance from the District Attorney's Assistant Al Schwartz, Vargas studies the public records on Quinlan's cases, revealing his findings to Gould and Adair. Quinlan arrives in time to overhear the discussion and angrily threatens to resignMeanwhile, Vargas sends his wife to a remote motel to remove her from the Grandis' unwanted attention. Unknown to him, the hotel is owned by Grandi. Grandi's family members terrorize Susie. Vargas becomes concerned when his attempts to telephone his wife at the motel are blocked. Per their agreement, Grandi arranges for Susie to appear unconscious from a drug overdose in a downtown motel, set up for Quinlan to "discover" her. Instead, Quinlan strangles Grandi there and leaves his corpse with Susie. When she wakes up, she screams for help but instead is arrested on suspicion of murder.Vargas confronts Menzies about Quinlan's history of "discovering" incriminating evidence in his cases. He goes to Susie's motel but cannot find her, discovering both that the motel is owned by Grandi and that his handgun has been stolen. He rushes back to town and enters a bar, where he confronts the gang members who attacked his wife. When they refuse to answer his questions, Vargas attacks them but is overwhelmed.
Schwartz informs Vargas that Susie has been arrested for murder. At the lockup, Menzies reveals to Vargas that he discovered Quinlan's cane at the murder scene, implicating him. Menzies agrees to wear a wire for Vargas. Quinlan is at ex-lover Tanya's brothel across the border. A loud player piano prevents recording, so Menzies lures Quinlan out. While they walk Vargas trails them, recording their conversation.
Quinlan confesses to Menzies that he planted evidence on suspects, but just the "guilty" ones. Quinlan hears an echo from Vargas' tracking device and suspects Menzies of betrayal. Quinlan demands that Vargas show himself and then shoots Menzies. Just as Quinlan is prepared to shoot the unarmed Vargas, claiming he'll plead self-defense, Menzies shoots Quinlan, then expires. Schwartz arrives at the scene and tells Vargas that Sanchez had confessed to the crime. Vargas is reunited with Susie. Tanya arrives and rues Quinlan's death.