Trent, a lone man, is running through the city of Los Angeles with no memory of who or where he is. All he can remember is that he escaped from his captors recently, and they are after him. Trent takes refuge in an alleyway and removes the glove on his left hand... revealing a glass hand embedded with circuitry. Only the thumb and little finger are present: there are slots for other three fingers. The computer is an artificial intelligence and Trent asks it who is following him. The glass hand doesn't know, but informs Trent that each pursuer is wearing a medallion, as is he. Trent's captors have come from the future to the present using a Time Mirror and the medallions, and that they can summon reinforcements at will. It then tells Trent that it must be complete to answer any remaining questions.
One of the captors, an alien humanoid with distorted features, pursued Trent. Trent turns the tables and ambushes it and yanks off his medallion. The alien disappears in a flash of light, and Trent drops the medallion another alien, Breech, tries to find his comrade and Trent captures him. Trent ties the alien to a gate and it agrees to tell him what he wants to know rather than suffer pain. The alien explains that he is one of the Kyben, and that they've taken on human form to come back to the 20th century and find Trent. The Kyben were at war with humanity, a thousand years in the future, and they want the glass hand because it holds all knowledge. The aliens have the three fingers, the computer's "lobes," which they obtained from Trent when they captured him as the last man on Earth. Breech informs Trent that the medallions hold them in the present, and if they are removed then the temporal tension yanks them back to their own time with lethal results. Trent forces Breech to reveal the location of the Time Mirror, at the Dixon Building, and then yanks off the alien's medallion.
As Trent takes the sewers to the Dixon Building, he questions the hand further. It tells him that when the Kyben attacked, Earth had no defense against them and humanity was conquered within a month. However, all 70 billion men, women, and children disappeared overnight, leaving Trent as the only remaining native on the planet. Scientists gave Trent his glass hand and the computer within. The Kyben captured Trent, who managed to escape and make his way through the Time Mirror to the 20th century. The glass hand reiterates that Trent must find the three missing lobes and destroy the Time Mirror to prevent the Kyben from sending more reinforcements.
Trent enters the Dixon building from below, and the computer explains that it was once a major business but has fallen on hard times and is now in the middle of a slum. The Kyben are using one of the offices as a front for their operations. As Trent starts exploring, the Kyben leader, Arch calls to him and informs him that Breech was under orders to tell Trent where they were so they could capture them. The Kyben enclose the building in an invisible force field, and Arch offers Trent his life if he surrenders. Trent checks the glass hand, which tells him that it is not yet able to tell him where the 70 billion future Earthlings are. It does know that they are in hiding and that Trent is the key to their release.
A Kyben soldier spots Trent and shoots at him, and Trent returns fire and kills him. He then ducks into a women's clothing office, C&E Biros Apparel, and turns on the light. He finds a woman, Consuelo Biros, hiding in the corner at the sound of the gunshots. Consuelo begs him to spare her, and Trent grabs her and holds her silent as the Kyben pass outside. He warns her that he is being hunted and they are both in danger, and Consuelo remains silent when he releases her. Another Kyben soldier enters, and Trent knocks it out. He tells Consuelo what he knows, and when she proves skeptical, he rips off the alien's medallion. Seeing it disappear, Consuelo realizes that Trent is telling the truth. She explains that she now works there as a cleaning lady and asks to leave, but Trent warns her about the force field. When he tells her about the Time Mirror, Consuelo suggests that he go to the basement to avoid the Kyben patrols. There is a blocked door leading out of the office
As the couple leaves, another Kyben, Battle, spots them. Trent knocks him out before he can sound the alarm and drags him into the office. As he prepares to rip off the alien's medallion, Consuelo asks if he has to kill Battle. Trent says that it's gone too far for anything else, and Consuelo explains that she deplores violence because her husband, Esteban, beats her after the failure of their business. Trent relents and interrogates the Kyben. Battle claims to be a patriot and refuses to talk, but quickly gives in. He tells Trent that Arch has one lobe, and Durn and Vaughn in the future have the other two pieces. When Consuelo pleads for Trent to spare Battle, the Kyben takes advantage of the distraction, runs to the door, and yells an alarm to his comrades.
As Battle escapes, Trent and Consuelo slip out into the hallway. As the Kyben soldiers close in, Trent asks the glass hand what to do. It tells Trent to let the Kyben kill him. He considers its advice and then steps out into the hallway. The Kyben gun him down, and Arch confirms that Trent is dead. He attaches the finger lobe and tells the others that when Vaughn and Durn arrive, they can attach the other two lobes and learn where humanity is hiding from the invaders.
The Kyben take Trent to the apparel office and leave him on a table. Arch locks up Consuelo as well, unsure who she may be. He tells his lieutenant that once they have the complete hand, they can use its knowledge to return safely to the future. Once she's alone, Consuelo asks the Virgin Mary for help. The glass hand activates in response to her question, and tells her to apply a moist warm cloth to three key points on Trent's head and neck. He comes back to life and the glass hand has Consuelo remove the cloth as Trent's temperature soars. Trent quickly stabilizes and determines that there are no bullet holes and no sign of the fatal wounds. When Consuelo tells him what the glass hand did, Trent wonders how powerful it will be once it is fully restored.
As Trent recovers, Consuelo tells him that Durn and Vaughn re coming. He consults the glass hand, which explains that the Kyben can use it when completed to find humanity, and learn how to safely return to their own time or survive permanently in the present. However, the glass hand still needs the second lobe to tell Trent where humanity can be found. It detects an energy drain and informs Trent that Durn has come through the Time Mirror. Trent leaves to search for Durn and takes Consuelo with him. Durn finds them first and knocks out Trent. When he prepares to kick Trent to death, Consuelo reluctantly rips the medallion off of his neck, killing him. He drops the lobe as he disappears, and Trent picks it up as he recovers.
As the Kyben close in, Trent takes Consuelo to the top floor storage room. He confirms that the force field is still present and tells Consuelo that she'll have to hide on the outside. She hesitates, afraid of heights, and retreats inside before fainting. Trent holds her and inserts the second lobe. The glass hand informs him that humanity realized that it couldn't defeat the Kyben directly. Instead they created a radioactive plague with a two-hundred-year life. The plague would wipe out all intelligent life on Earth, including themselves, so humanity used a device to translate the entire race into electrical impulses stored on a strand of gold-copper alloy wire. As Consuelo wakes up, the glass hand tells Trent that humanity entrusted Trent with the wire. However, it lacks the knowledge of where the wire is hidden, and needs the third lobe to tell Trent that final piece of information.
Battle reports to Arch that Vaughn is coming through the Time Mirror, and has the final lobe.
Consuelo worries that Trent is becoming tired, but he admits that he doesn't seem to have grown tired despite the fact he hasn't slept since he escaped the Kyben. He wants to get her to safety, but Consuelo says that she'd rather stay. She tells Trent that she's fallen in love with him, but he says that he can't return her feelings. Consuelo says that it doesn't matter as long as one of them loves the other. She starts to kiss him, but Trent turns away and tells her to hide outside.
Once Consuelo reluctantly goes outside to the roof, Trent goes back downstairs and finds Arch. He captures the alien leader and forces him to reveal the Time Mirror's location or lose his life. Arch gives in, but boasts that Trent can't win. Trent rips off the medallion anyway and then imitates Arch's voice, telling the Kyben that "Arch" is okay. Battle warns that Vaughn is coming and that there's been a report of trouble in the future. Trent slips into the outer office and watches the inner office containing the Time Mirror. Another Kyben, Budge, comes through the Time Mirror and informs Battle that he has come ahead to make sure it's safe for Vaughn. He informs Battle and the remaining Kyben soldiers that a plague is decimating their invasion force in the far future. Budge claims that he came forward to make sure they recover the glass hand and use its knowledge to stop the plague. He angrily denies Battle's accusation that he tried to escape to the 20th century to avoid the plague.
As Vaughn starts to appear, Budge prepares to take the lobe in case something happens. He takes it from Vaughn, and Trent smashes into the inner office. He rips off the medallions from two of the Kyben soldiers, and then struggles with Budge. The lobe falls to the floor and Battle grabs it. He runs outside as Trent knocks out Budge, realizes he can't escape in time, and tosses the lobe down a mail chute. He stuns Trent and takes the elevator down. Recovering quickly, Trent leaps down the stairs and manages to beat the elevator to the ground floor. He shoots open the mail box and gets the lobe, and then flees to the attic as Battle and the last Kyben soldier pursue him. Trent inserts the final lobe into the glass hand and then jumps down from the rafters to knock out the second soldier. Seconds later, Battle grabs Trent and smashes him through the window. As they struggle several stories above the street, Consuelo intervenes for a second time and rips the medallion off of Battle's neck while begging God to forgive her.
Trent tells Consuelo that he destroyed the Time Mirror, and that she should go downstairs while he disposes of the second soldier for good. Once he does, he meets her in the office and asks the glass hand to reveal humanity's location. It tells Trent that the wire is shielded and wrapped around Trent's control solenoid. Trent has no idea what it means, and it tells him to push back all three fingers. When Trent does so, a panel opens in his chest, revealing wires and circuitry. The glass hand explains that Trent is a robot, programmed by humanity to believe that he is a human. As a robot, it was a simple matter for the glass hand to activate the repair sequence and "resurrect" Trent.
Stunned, Trent reaches for Consuelo, but she withdraws from him with a look of mingled horror and pity. She walks out of the building and Trent watches her go. As an immortal robot, he now has to wait 1,200 years until Earth is safe and he can release humanity... a humanity he was never a part of and never will be.