In the ghost town of Morgue, CA, Major Roger Hill drives Professor Stephen Grave up to a seemingly abandoned hotel. Hill activates a buzzer and a sentry takes his papers and goes back inside. While they wait, Grave looks around. The major looks up at the sky and Groves asks if he saw "them." Hill says it was probably a bird and figures that their visitors may come in the daytime since they have nothing to fear. Groves asks why they're allowing it to happen, and Hill says that they have no choice. He explains that they were told when the penal ship would land and instructed to stay out of its way "or else." When Groves wonders what the "or else" was, Hill says that they wanted a non-violent sequence but promised to attack if anything went wrong. The major thinks that "they" either superhuman... or subhuman.
The sentry ushers them in while another soldier drives the car away. Inside, the military has set up a command center General Hart greets Grave, who explains that he would have been there two days ago but someone decided that he should be there to document the situation. Hart explains that they're dealing with a new form of battlefield and shows Grave the radar system. He explains that the Zanti are in Earth's atmosphere but haven't contacted them yet. He then introduces Grave to the rest of the team, explaining that he will be functioning as an interplanetary historian and observing everything they do.
Hart takes Grave to his desk and examines the old-fashioned typewriter that he uses. Grave talks about how his father, a war correspondent, was far more sentimental. The professor figures that his father wouldn't have been sentimental about aliens. Hart suggests that the Zanti might be friends but Hill speaks up, noting that friends don't use coercion on each other. Grave admits that he's read the transcriptions and agrees with Hill, but Hart doesn't want war. According to the psychological analysis, the Zanti are perfectionists and don't want war anymore than Earth does. The Zanti want to use Earth as a place to exile their misfits and criminals, and have said that they are psychologically incapable of executing their own species. Hill points out that they don't send their criminals to other planets, but Hart notes that they don't have interplanetary travel.
The general shows Grave a map of the area and explains that they've assigned patrols to circle the isolated area. The sentries will kill anyone who stumbles into the area allocated to the Zanti... and keep the Zanti inside the area. When Grave wonders what will happen if the Zanti leave the area, Hart explains that they have a missile base ready to fire if the Zanti ship deviates from its course.
The Zanti contact the base and Hart hails them. He gets no response from the penal ship and the technicians confirm that the ship hasn't landed yet. Hart repeats his hail and the Zanti finally respond in their own language. The staff translates and the First Regent of the penal ship informs them that they are ready to land in the designated area. The Regent points out that there are weapons aimed at the area and Hart explains that the sentries are there to assure their privacy. In response, the Zanti say that they will land but warn that there will be total destruction of anyone that invades their privacy. Hart assures Grave that the sentries will kill anyone who enters the area.
Ben Garth and his girlfriend, Lisa Lawrence, drive up to a gate into the area and Corporal Delano refuses them entry. After a moment, Ben backs up the car and then runs the gate, running the sentry over. He continues into the desert are while nearby, the Zanti ship touches down. The car's radiator finally blows and the car grinds to a halt.
Hart, Grave, and Hill go outside to watch the Zanti ship, some eight foot in diameter, lands in the distance. Hill believes that they'll try to break out. Hart reminds them that the Zanti wardens guaranteed maximum security, but Hill points out that all prisons guarantee that. An air police officer reports that they've spotted a car on the radar.
Ben and Lisa settle in and Ben assures her that he'd never kill a reasonable man. Lisa finds the idea amusing but Ben insists that the guard wasn't reasonable. She admits that she ran away from her husband and went with Ben because she knew he was a psychotic and she needed the excitement. Ben says that they need water for the engine and tells her to pick up the money she stole from her husband. Lisa refuses and gets out of the car, saying that she's going to go home and face up to what she's done. As she starts to walk away, Ben calls her back and she reluctantly returns. He orders her to pick up the money and she does so, and Ben warns her that if she goes home then her husband will make her tell him the whole story and then the police will lock him up. Resigned to her fate, Lisa sits back down.
The air police bring the sentry's corpse back and Hart orders the missile base to red alert. He warns Hill that the Zanti will listen but not respond to them.
Ben spots the Zanti ship on a cliff.
Hart calls the Zanti and tells them that they have an unauthorized entry. He gets no response.
While Lisa hides in the car and rolls up the windows, Ben climbs up the hill. He's unaware that something in the ship is watching him as it listens to Hart's message. Hart says that he is sending a party in to remove the intruders and assures the Zanti that they are holding to their agreement.
Ben approaches the ship and peers in through a viewing port. A hatch opens, knocking Ben down and injuring his back in the fall, and a foot-long insectile creatures emerges. As it crawls onto his hand, Ben begs it to get off of him. At the command center, the officers listen as Ben screams in terror.
A bored Lisa turns on the radio and listens to a news report about UFO sightings in the area.
Hart talks to his superiors and advises them not to send anyone else in for fear of further arousing the Zanti's suspicions. They order him to send men in if they don't hear from the Zanti in the next fifteen minutes. Hill warns that the Zanti could conquer the planet in fifteen minutes.
Lisa gets out of the car and calls to Ben, but gets no response.
Hart worries about the consequences of setting off a war and Grave suggests that they send an emissary in to discuss the situation personally. The general worries what will happen if the Zanti don't respond, and Grave offers to go himself. The scientist is eager to experience history for once instead of reading about it, and Hart reluctantly agrees to let him go.
Lisa climbs up the hill and finds Ben's corpse.
Grave takes a jeep and drives off into the Zanti area.
Lisa approaches Ben's corpse and rolls him over. The Zanti is clinging to his back and Lisa backs away in terror as it crawls toward her.
Hart sends a message saying that Grave will wait for an hour to receive permission to approach. The general assures the Zanti that they mean no harm.
Lisa runs for her life and finally collapses in exhaustion near the car. The Zanti continues pursuing her and she gets up and locks herself into the car. It climbs up on the window, shrieking.
Grave pulls up a few minutes later and hails the Zanti with a bullhorn. He tells them that he will wait one hour for their signal to advance. Meanwhile, Lisa opens the car window when she realizes that the Zanti has retreated. She gets out only to find the Zanti waiting for her on the roof. Screaming, she runs away.
The command center overhears a broadcast from the Zanti ship. A different Zanti says that the Regent has gone after "her" and it's time for them to take advantage of the situation. Hart wonders who they're referring to and radios Grave. The general tells him that there's a woman out there and they're going to have to destroy the ship because the prisoners are making a break for it. Before he can leave, Grave spots Lisa and calls to her. Confused by the echoes, Lisa runs into a dry gulch and collapses. The Zanti closes in on her and she says that she's not afraid to die. Grave arrives and crushes the Zanti with a boulder. Lisa rambles on about how she tore her life up and Grave takes her to his jeep.
The Zanti ship lifts off and Grave calls Hart and explains that he had to kill one of the Zanti. Hart says that they've received no communication from the Zanti. Hill prepares to call the White House, concerned that the Zanti are invading, and Hart suggests that the aliens may be going back to their home planet. The major doesn't believe it and tells Hart to advise the President to destroy the Zanti ship. Before Hart can decide, they confirm that the ship is heading their way.
Grave pulls up to the hotel and takes Lisa inside as the Zanti ship comes down atop the building. Hill tells Hart that the ship is now too close to blow up. The Zanti emerge from their ship and start climbing down. Hill panics, grabs a rifle, and runs outside. He fires at the aliens but one of them jumps onto him and bites him while the others close in.
Grave tells Hart to use hand grenades and flamethowers. They open the rear door and Hill stumbles in. Grave beats it to death while more of the Zanti swarm into the building, overwhelming anyone who gets too close. The soldiers shoot and club the aliens, blowing them up with grenades when they can.
Finally all of the Zanti are dead. Hart wonders how the Zanti high command will destroy them now that they've killed the prisoners. The Zanti government contacts Hart and informs him that they will not retaliate. They never intended to because they knew that humans couldn't live with such aliens in their midst. The Zanti realized that the humans would destroy the prisoners as well as the guards, who were of the same spoiled persuasion. Since they couldn't execute their own species as a matter of principle, they chose Earth knowing that the practiced human executioners would do it for them.