TV journalist Lee Carter witnesses the assassination of U.S. senator and presidential aspirant Charles Carroll atop the Seattle Space Needle. The apparent shooter, a waiter, is chased to his death, while the actual killer, disguised as a waiter, vanishes in the chaos. An investigation by an official panel attributes the killing to a single man who acted alone.
Three years later, Carter visits ex-boyfriend Joe Frady, an investigative newspaper reporter. She tells him that six witnesses to the assassination have since died and she fears she will be next. Soon after, Carter is found dead of a suspected drug overdose.
Feeling guilty about disregarding Carter's pleas and suspicious of her sudden death, Frady goes to the small town of Salmontail to probe the drowning death of Judge Arthur Bridges, another witness to Carroll's assassination. The local sheriff, Wicker, offers Frady his assistance and escorts him to the dam where Bridges met his fate to investigate. As its floodgates open, Wicker pulls his gun on Frady.
After a desperate struggle in which the sheriff drowns, Frady escapes in the sheriff's squad car and races to the sheriff's home. There, he uncovers documents from the Parallax Corporation, an organization recruiting "security" operatives. Later, Frady contacts a local psychology professor, who assesses a Parallax personality test taken from Wicker's desk. He deems it a profiling exam to identify homicidal psychopaths.
Austin Tucker, an aide to Carroll and witness to his assassination, agrees to meet Frady. He reveals there have been two attempts on his life since Carroll's murder. Shortly after, a bomb goes off on his sailboat, killing Tucker and his bodyguard. Frady is also presumed dead by the authorities, but swims ashore and tells his editor, Bill Rintels, he will use his apparent death as cover for applying to Parallax under an alias.
Days later, Parallax official Jack Younger contacts Frady to tell him Parallax has accepted him for training. On his first visit to its Los Angeles headquarters, the corporation measures Frady's reactions as he watches a montage of disturbingly edited still pictures that juxtapose pro- and anti-American attitudes. After leaving the offices, Frady trails a Parallax operative to Los Angeles International Airport, where the man sends a bomb aboard a passenger jet in the checked baggage.
Frady boards the plane and spots a United States senator who, like the ill-fated Carroll, is now considering running for president. Frady secretly passes a note to a flight attendant about the bomb, and the jet returns safely to Los Angeles and is evacuated just before it explodes.
Back at his apartment, Frady is confronted by Younger about his fake alias, but mollifies him with a pre-planned cover story. Later, at the newspaper office, Rintels listens to a recording of the conversation between Frady and Younger. He then places it in an envelope with other recordings. That evening, he is poisoned by the same Parallax assassin and the recordings vanish.
Frady returns to Parallax headquarters and follows the same Parallax operative to a huge exhibit hall in the Los Angeles Convention Center, where a dress rehearsal is being held for a political rally for another presidential aspirant, Senator George Hammond. Frady chases the operative and watches from the hall's catwalks as Hammond drives toward the exit in a golf cart and is fatally shot by an unseen sniper.
Frady is spotted on the catwalk and falsely identified as the shooter. Realizing too late he has been set up as a patsy, he attempts to flee, but is killed by Parallax security men. Six months later, another official investigative committee reports that Frady was a paranoid lone gunman who killed Hammond out of a misguided sense of patriotism.
