A United States diplomat causes international shock by detonating a bomb at a peace conference in Venice, killing all the attendees, including himself. Former CIA agent Bill Fenner, played by Robert Vaughn, is brought back to find his ex-wife, who was hired by Karl Boehm's character, Wahl, a prominent enemy agent, to befriend the American diplomat. He is also tasked with recovering a report written by a political scientist, Dr. Pierre Vaugiroud, played by Boris Karloff, which explains the reason for the bombing.
As Fenner searches for the report, he becomes the target of Communist agents, assassins and blackmailers. His ex-wife is killed as well. Wahl, eager to obtain the report himself, kidnaps Fenner to his secret lab and tries to turn him into a mind-controlled robot using drugs. The attempt fails, and to save both of their lives, Vaugiroud persuades Fenner to reveal the report's location. Fenner discovers that Vaugiroud is also being controlled by drugs under Wahl's influence. After killing Wahl and escaping the lab, Fenner rushes to another peace conference to prevent Vaugiroud from causing another bombing.