Scruffy derelict Cameron Vale (Stephen Lack) cruises through a shopping mall scavenging food and cigarettes. While eating leftovers left behind at a table at the mall food court, two rude middle-aged women notice him and openly talk (very out loud) about how disgusted they are by his appearance. As Vale angrily glares at them, one of the women suddenly goes into convolutions and falls to the floor shaking. Two men in trench coats appear and give chase, telling their contacts on their portable radios that they have found a "scanner". As Vale runs, another trench coat man appears and shoots him in the hand with a dart gun. Vale runs, pursued through the mall and finally collapses to the floor.
Vale wakes up strapped to a bed in a warehouse. Dr. Paul Ruth (Patrick McGoohan) approaches Vale and tells him that he is a derelict because he is a "scanner". Ruth ushers in a group of people whose voices grow in volume but their lips do not move. Vale thrashes on the bed as he cannot block out their thoughts. Ruth administers an injection and the voices fade away.
At a lecture at the ConSec corporation, the audience watches a demonstration of "scanning". The demonstrator tells them that he is a scanner who can scan peoples thoughts and asks for a volunteer display his powers. A man (Michael Ironside) steps forward and offers to volunteer. The demonstrator asks him to think of something specific which the demonstrator will scan his thoughts to discover. The scanner soon directs his focus, but he is soon goes into great pain, shaking in mountainous agony until his head literally explodes, showering blood, brain and skull matter all over the stage. Panic erupts in the auditorium as people flee towards the exits. Security guards grab the volunteer saying that have another scanner among them. They take the volunteer away at gunpoint to a stairwell where Dr. Gatineau is told to administer a shot of "ephemeral" but the scanner makes him inject his own hand instead. As the volunteer is driven to "the factory" in a car with two security guards, a second security car suddenly swerves into a wall and explodes. The lead car stops to help. The man smiles as the chief of security shoots the two guards and then himself. The sinister scanner then escapes into the night.
The next day, ConSec Chairman Trevelyan address the board of directors saying: "Last night, we at ConSec chose to reveal to the outside world our work with those telepathic curiosities known as scanners. The result: six corpses and substantial loss in credibility for our organization". The new director of security, Breadon Keller, wants the scanner program shut down. Dr. Ruth points out that the man responsible is a scanner. His own scanners have all been lost to a powerful underground organization led by the same man: Darryl Revok, the volunteer from the other night who has been identified from the security images on CCTV cameras. Ruth proposes infiltrating Revok's organization. Keller scoffs at the idea as foolish, but Ruth says that he has a "very special" new scanner who will help them.
Ruth shows Vale a film of a younger Revok in a psychiatric institute, a hole drilled in his forehead. Revok says he did it to "let the people out". Revok grows quite agitated and his pacified by two orderlies. In a local metro station, Keller informs an unseen man about Vale.
Ruth stages a test. Vale makes Yoga master Dieter Tautz's heard beat faster, despite his efforts to retain control. In agony, he begs Val to stop. Ruth is astonished by Vale's power. His next task is to find artist Benjamin Pierce, a possible link to Revok's underground organization.
Vale goes to a local art gallery where he makes the owner, Arno Crostic, an offer for one of Pierce's bizarre sculptures. When told it is impossible for him to meet the artist, Vale scans Crostic for Pierce's location, and is in turn scanned by a woman. When Vale turns around she is gone.
At his rural workshop, Pierce is angered by Vale's arrival. Sitting inside a giant sculpted head, he refuses to answer questions. Suddenly, four assassins burst in and open fire, eventually wounding Pierce mortally. Vale's scan flings them across the studio. The dying Pierce's thoughts give Vale a name: "Kim Obrist".
Vale finds Kim (Jennifer O'Neill), who is the young woman from the gallery, but Revok and his men have staked out her house. As her group of scanners sits in a circle, minds linked, two of Revok's assassins burst in and start blasting. The scan throws them into a wall they burst into flames. The survivors make their getaway in an old bus, but it is ambushed by more of Revok's men and plows into a record shop. An assassin follows, only to be paralyzed by Vale's scan, which searches for something that will lead them to Revok. The man produces a vial with a "Biocarbon Amalgamate" logo.
At the Biocarbon Amalgamate building, there is a sterile lab environment where workers wear white fallout suits. One of them is Vale who has snuck into the building and watches Revok direct operations. A computer image shows a delivery of ephemeral. Access to the program, called "Ripe" is limited to ConSec terminals only.
Vale calls Ruth, saying he's bringing in an informant from Revok's group. At the metro station, Revok instructs Keller to kill Ruth if he discovers anything. Meanwhile, Vale and Kim arrive at ConSec, and are injected with ephemeral. Keller takes Kim for interrogation. Vale protests, but Ruth confides their injections were harmless. Vale says that Biocarbon Amalgamate manufactures ephemeral for Revok, and sends it out in tankers. The destinations are on the ConSec computer, meaning a traitor is working with Revok. Ruth tells Vale to access the Ripe program as he does not have computer clearance. Vale says neither does he. Ruth tells Vale: "No, but you do have a nervous system... and so does a computer".
In the interrogation room, Keller knows that Kim is not from Revok's group. He pulls a gun on her to shoot her, but is overpowered by her scan. As Kim escapes from the room, Keller sounds the alarm. Vale runs to find her, and Keller shoots Ruth. Vale and Kim use their scan to escape from the guards.
On a rural road, Vale mentally downloads the Ripe program via a gas-station payphone. Keller is informed that someone is accessing the program, and at gunpoint, orders the technician to self-destruct the computer. Vale is hurt by the destruction of the computer, but his wave of destructive energy surges down the phone line, causing an explosion that kills Keller and the technicians at their computers. The overhead power lines break and fall, setting the nearby gas station on fire and it too explodes. Vale and Kim head for the nearest doctor on the Ripe program list.
In Dr. Frane's waiting room, Kim is scanned by a pregnant woman's unborn child. She is felled by a dart. Vale carries her outside where Revok and his men wait and they too shoot him with a drugged dart.
Vale wakes up in Revok's office where he reveals to Vale that Paul Ruth was their biological father. He gave ephemeral to his pregnant wife, whom was one of his many patients in the scanner project, so he could observe on the side effects on his own children. Revok wants them to leave the "whole generation of scanner soldiers just a few months away from being born." Vale refuses to join Revok's side and strikes him in the face, who launches a furious mental assault as both viciously scan each other causing each other's blood vessels to expand. Vale seems to be losing the mind battle as Revok's ravaging scan causes his body to bleed, corrode and burn. His eyes seem calm until they are blown out. Revok summons a final intense burst as he seems to go into convulsions as his own body seems to be affected.
A little later, Kim wakes up in the waiting room and enters the office and finds Vale's charred corpse still sitting in its chair. Kim looks distraught but senses something as she says "Cameron?" In the corner of the room a figure huddles under a coat which is thrown off to reveal Revok. Upon rising and walking towards Kim, Revok has now as Vale's blue eyes. Speaking in Vale's voice Revok says: "It's me, Kim. Cameron, I'm here. We've won. We've won..."