Major Roger Hill drives Professor Stephen grave into the ghost town of Morgue, CA. They go up to a seemingly abandoned hotel and Hill activates a hidden buzzer. A sentry takes their papers and goes back inside. While they wait, Grave and Hill watch the sky and Grave wonder if Hill saw "them," noting that it's daytime. The major figures that they could send their penal ship during the day because they have no reason to worry about anything that Earth's military can do. Hill points out that the aliens they're dealing with are too powerful and that they were told to let the penal ship land in an isolated area "or else." The professor wonders what the "what else" was, and Hill says that the aliens promised to attack if anything went wrong. The major says that they may be dealing with superhumans... or subhumans. Grave says that they may simply be non-human.
The sentry ushers the two men into the command center set up inside of the abandoned hotel. General Maximillian Hart greets Grave, an old friend, and is glad to have him there to document what will happen. The general says that they're dealing with a new battlefield and shows Grave their banks of electronic system. The radar shows that the aliens, the Zanti, are in Earth's atmosphere but haven't established contact yet. Hart introduces Grave to the rest of the team, explaining that he will observe and document everything that happens.
As Hart takes Grave to the desk set aside for him, Grave says that his father, a war correspondent was a sentimental man but even he wouldn't have been sentimental about aliens. Hart is willing to believe that the Zanti are friendly, but Hill speaks up and says that friends don't coerce each other. Grave has read the transcriptions of the Zanti communications and agrees with Hill. However, Hart insists that he doesn't want war and doesn't believe that the Zanti, who are perfectionists, want war either. The Zanti want to exile their criminals and misfits to Earth and are psychologically incapable of executing their own kind. Hill is still suspicious, pointing out that they don't send criminals to other planets, but Hart notes that they don't have the means to do so... yet.
Hart shows Grave a map of the desert and explains that they've isolated the entirely area and placed armed patrols. The patrols will kill anyone who enters the area, but are also there to make sure the Zanti criminals stay to their designated area. If the Zanti try to leave, Hart assures Grave that they a nearby base has missiles aimed at the area and will destroy the Zanti ship if it deviates.
The Zanti's First Regent of the penal ship finally contact the military base and, after translation, tell Hart that they are ready to land. It has detected the weapons aimed at the area and Hart tells them that the sentries are there to assure their privacy. The First Regent agrees to land but warns Hart that they will destroy anyone who enters the area.
Ben Garth and Lisa Lawrence drive up to the gate around the area. When the sentry, Corporal Delano, refuses to let them through, Ben runs him over and breaks through the gate. Ben continues driving into the hills until the radiator blows out on the car. As Ben and Lisa get out of the car, the Zanti ship--the of a small closet--touches down nearby.
The military staff goes outside to watch the Zanti ship land. Hill believes that the criminals will try to escape, pointing out that despite the Zanti's assurances, prisoners have been able to escape prisons in the past. They're interrupted when an officer informs them that a car has been detected in the isolation area.
Ben and Lisa relax in the car and Ben insists that he only kills unreasonable men like the sentry. Lisa doesn't believe him and admits that she ran away from her husband and went with Ben knowing very well that he was psychotic. She wanted the excitement and stole the money from her husband to finance her escape. Ben tells Lisa to take the money out of the glove compartment and get out, but Lisa refuses and says that she's going to go home and face up to what she's done. He calls her back and Lisa reluctantly comes back and gets the money out. Ben insists that she can't go home because if she does then her husband will make her tell him the whole story, and he'll find out about Ben's involvement. Once that happens, the police will soon arrest him. Realizing she has no choice, Lisa sits down in the car.
The soldiers bring Delano's corpse back and Hart puts the missile base on red alert. The general prepares to call the Zanti First Regent but warns Hill and Grave that the Zanti will not respond to them.
Ben looks around and sees the Zanti penal ship on a nearby cliff.
Hart calls the Zanti and tells them that there has been an intrusion into the isolation area, but the First Regent doesn't respond.
Lisa rolls up the windows and locks the car doors while Ben climbs up the hill to the ship. As he approaches it, a Zanti watches him approach and listens to Hart's transmission. Hart says that they are keeping their side of the agreement and are sending a patrol to remove the intruders.
Ben tries to see inside the ship and a portal on the side springs open. Shocked, Ben falls back and injures his back as he tumbles down the hill into a crevasse. As he stares in horror, a Zanti emerges--an eight-inch-long ant-like creature with a human face. It crawls onto Ben's arm and he screams in terror as the alien crawls toward his face. At the base, Hart and the others listen to Ben's screams.
A bored Lisa listens to the radio as a newscaster talks about the recent batch of UFO sightings in the area.
Hart calls his superiors in Washington and warns them that sending anyone in could further antagonize the Zarbi. They tell the general to send a patrol in if they don't hear from the Zanti in the next fifteen minutes. Hart has no choice but to wait but Hill points out that the Zanti could conquer Earth in fifteen minutes.
When Ben doesn't return, Lisa gets out of the car and calls to him. However, she gets no answer.
Hart worries about starting a war and Grave suggests that they send an emissary to talk to the aliens: Grave himself. The professor is eager to actually experience history rather than simply read about it and Hart reluctantly lets him go.
Lisa climbs up the hill and finds Ben's corpse wedged in the crevasse.
Grave drives toward the Zarbi spaceship in a borrowed jeep.
Lisa reaches for Ben's corpse and rolls it over, and the Zanti crawls off of his back. The woman backs away in terror as the alien crawls toward her.
Hard transmits a radio message telling the Zanti that Grave will wait for an hour to wait for permission to approach, and assures the aliens that the professor means no harm.
Lisa runs away from the Zanti and collapses, exhausted, next to the car. When the Zanti arrives, she locks herself into the car but crawls up the window, shrieking.
When Grave pulls up near the ship, he takes out a bullhorn and tells the Zanti that he'll wait one hour for their signal. Meanwhile, the Zanti stops shrieking and disappears, and Lisa hesitantly opens the car window. When she gets out, she sees the Zanti waiting for her on the car roof and runs away, screaming.
At the command center, Hart and his men overhear one of the Zanti prisoners speaking on the radio, saying that the Regent has left to pursue the woman and it's time for them to escape. The general radios Grave and tells him that there is a woman out there, and informs the professor that they're going to have to destroy the ship now that the prisoners are free. As Grave prepares to leave, he sees Lisa at a distance and calls to her. Confused by the echoes, she wanders into a dry gulch and collapses as the Zanti Regent approaches her. Resigned to her fate, Lisa turns to face it and says that she's not afraid to die. Grave arrives and crushes the Regent beneath a boulder, and takes a hysterical Lisa to the jeep.
The Zanti penal ship lifts off and Grave calls to tell Hart what happened. The Zanti haven't communicated with Hart and Hill prepares to call the White House for orders. Hart insists that the Zanti may simply go back to their home planet. However, the major doesn't believe it and advises his superior to advise the President to destroy the ship. Before Hart can decide, the radar operator picks up the approaching Zanti ship.
Grave pulls up to the command center and brings Lisa inside, and they listen as the Zanti ship comes down on top of the building. Hill warns Hart that the ship is now too close to safely destroy. When they hear the Zanti climb down the walls, Hill panics, grabs a rifle, and runs outside. The alien prisoners are climbing down the walls and he starts to shoot them, but one leaps on him and starts biting him.
Grave advises Hart to use hand grenades and flame throwers. When they open the rear door to the arsenal, Hill stumbles in and Grave kills the Zanti on him. More of them swam inside, overwhelming several soldiers through sheer numbers. The soldiers return fire, clubbing and shooting the Zanti until they have wiped out everyone one of the aliens.
In the aftermath, Hart waits for a signal from the Zanti, worried that they will conduct a massive retaliation. However, when the Zanti contact the command center, they tell Hart that there will be no retaliation. They never intended for the prisoners to live. Since they couldn't bring themselves to execute their own kind, they sent them to Earth, knowing that the humans couldn't tolerate aliens among them. The Zanti knew that the humans, practiced executioners, would kill the prisoners for them.