While crossing the border from Mexico to the U.S. wealthy businessman Linnekar and his stripper girlfriend are killed when a time bomb explodes in their car. Since the bomb was planted in Mexico, Mexican narcotics agent Mike Vargas (Charlton Heston in brownface) teams up with corrupt American cop Hank Quinlan (Orson Welles) to solve the murders. Meanwhile, Mike's new wife Susie (Janet Leight) is menaced by members of the Grande crime family whom Mike is after. Susie decides to accompany Mike to the U.S. as he becomes part of the investigation; they are tailed by Uncle Joe Grande.
Quinlan and Mike question the suspect in the murders, Sanchez, who is living with Linnekar's daughter Marcia. Sanchez had worked for Linnekar's construction company and had access to dynamite stolen from the company and used in the car bomb. Quinlan plants evidence on Sanchez and Mike accuses him of framing Sanchez. Quinlan and Grande secretly team up to frame Mike. One of the Grandes leads a gang and kidnaps Susie from her motel, dopes her up with heroin and marijuana and holds her in Rancho Grande.
With the help of Schwartz, who works in the District Attorney's office, Mike finds that Quinlan had bought dynamite. He also finds that incriminating evidence has mysteriously appeared in Quinlan's other cases. Mike accuses Quinlan of planting evidence and Quinlan accuses Mike of being a drug addict.
Learning that Susie's been kidnapped by the Grandes, Mike drives back to Mexico to find her. In a hotel in Rancho Grande, Uncle Joe has Susie doped up. Quinlan strangles Uncle Joe to death. Susie is arrested for drug possession and Uncle Joe's murder. Mike accuses Quinlan of framing Susie for murder to Menzies. Menzies shows Mike an article from the crime scene--Quinlan's cane. Menzies agrees to wear a wire and to get Quinlan to confess Uncle Joe's murder. Quinlan shoots Menzies and then tries to kill Mike, but in a final act, Menzies shoots Quinlan dead and then dies. Mike is reunited with Susie.