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When Benson, a U. S. narcotics investigator, is killed in the Iranian desert after offering to buy opium from a nomadic tribal chief, the United Nations sends agents Lincoln and Jones to smash the dope ring. Their trail leads first to Tehran, where they meet Linda, who, after claiming to be Benson's widow, disappears. The agents plot with U. N. scientists to intercept a cargo of opium, mark it with radioactive tracers, and use Geiger counters to follow it through underworld channels to the drug traffic center. For a while the agents maintain contact with the cargo but then lose it. Eventually the Naples police report having seized the cargo, which is traced to Marco, a wealthy playboy in Monte Carlo. Lincoln finds Marco aboard his yacht, hosting a party attended by Linda, and he questions Marco's wife, Monique, who is a drug addict. The following morning Lincoln's dead body is found, and Jones continues the pursuit alone. He tracks Marco to a Paris-bound train, where he again meets Linda, and she reveals herself to be a U. N. agent. After escaping an attack by Marco's henchmen, they return to Marco's yacht and succeed in implicating him in both the murder and the narcotics ring.