Three years after the third expedition, fleets of colony ships head for Mars. A reporter interviews General Halstead, who says that they don’t anticipate encountering any Martians since they were wiped out by chicken pox.
Colonel Wilder is appointed the chief coordinator of Mars and hopes to minimize Man’s impact on the planet. In six months a dozen communities are placed down and Mankind begins giving the Martian areas human names derived from the original expedition members. One canal is named Lustig Creek after David Lustig, who disappeared with the others on the second expedition.
David’s parents Leif and Anna are among those colonizing Mars. When the first rain comes, the Lustigs emerge from their home briefly to enjoy it and then go inside to warm themselves at the fire. Leif thinks of their son and Anna asks him not to bring it up again. She’s happy just to be there with her husband and Leif promises not to speak of it any more.
Later that night, Leif is in bed next to his wife. He wakes up when he hears someone whistling outside. Anna doesn’t hear anything and goes back to sleep but Leif goes to the window and looks outside. He sees a dim figure on the shore and goes out to get a better look, and sees his son David. He calls Anna out but she insists that it can’t be their son and tells him to go away. Leif tells her to stop but she runs. He calls to the figure, saying that he’ll leave the door unlocked if he wishes to come in and warm himself. The figure doesn’t respond and Leif goes back inside. Anna tells him to lock the door but he doesn’t. When he gets back into bed, Anna hugs him and Leif tells her to come to sleep. However, he wonders if it really is David before dozing off.
A few hours later, Leif hears someone walking in the next room. He ignores the sound and tries to sleep.
The next morning, Leif wakes up first and goes out into the living room. There’s no sign that anyone was there and he assumes that he’s losing his mind. However, when he comes out later for breakfast, David casually walks in and greets him. Leif asks if he’s alive and David asks why should he be otherwise. He refuses to answer Leif’s questions and tells him that he should accept him and not question him. When Leif worries that the shock will upset Anna, David says that he sang to them and it should make things easier for her. Anna comes out and is entirely comfortable with their missing son being there.
Once they’re alone, Leif asks who David really is. David tells him again not to question and begs him to accept him as he is. When Leif persists, David walks outside. Anna comes back in and Leif asks if she remembers how the second expedition never returned. She does, but doesn’t understand what it has to do with David.
Wilder watches as a group of missionaries arrive to teach God’s truth to the colonists. Among them are Father Peregrine and Father Stone, who offer a brief prayer upon their arrival. They have supper in First Town with Wilder and his wife and two children. Robert and Marie. Peregrine asks about a rumor he heard that a prospector broke his leg in the mountains, and spheres of light apparently carried him to safety. Stone warns that the rumors haven’t been substantiated, and Wilder says that there’s no scientific evidence of any life on Mars. As they have coffee, Peregrine talks of exchanging ideas with the Martians. Wilder assures him that he can see the ruins the next day and explains that the colonies are like towns in the Old West and they need missionaries.
The next morning, Wilder takes Peregrine and Stone to the ruins. Stone worries that the colonization effort is going too fast and Wilder admits that he wishes he could slow it down. He feels responsible and tells them about how Spender said there were a few Martians left. Given the rumors of Martian life, Wilder now wonders if a Martian may have taken over his friend and spoken through him.
They continue to the ruins where archaeologists are dismantling the structures for shipment back to Earth. Father Stone doesn’t see a problem with them taking the structures back for study, since the Martians are gone. Wilder admits that he looked for life but couldn’t find anything. He offers to take them back to town and Stone is eager to accept, but Peregrine says that they want to walk and see more of the land. Wilder gives them directions and says that he’ll send a vehicle back for them just in case.
Once Wilder leaves, the missionaries start walking across the land. Peregrine soon leaves the road, insisting that they’ll be all right and First Town is just over the next hill. As Stone continues to complain, they continue into the hills and become lost. Peregrine, unconcerned, suggests that they call out to the Martians and see if they get a response. Stone realizes that Peregrine has brought them out there to find the Martians and insists that they are needed in the towns. Before Peregrine can respond, three glowing balls of blue light descend from the sky. Stone insists that it’s the work of the Devil and starts to run, but Peregrine grabs him and then walks toward the creatures. He holds up his cross and says that they come with God, but the creatures start to drift off.
Peregrine yells after them, his voice setting off a landslide. As the two missionaries cower in fear, one of the creatures returns, surrounds them, and takes them to safety. Peregrine and Stone realize that they’re safe as the creature depart. Stone insists that they ran but Peregrine believes that the creatures saved their lives, proving that they have free will and souls. He tells Stone that he will stay the night and refuses to hear any argument. Stone helps him start a fire and Peregrine suggests that they try to prove that the creatures know sin and thus have morals and intellect. He points out that the Martians probably killed the first two expeditions, proving that they sinned. Stone questions if he is thinking of the Church or of personally converting the Martians, and warns that would be a sin of pride. Peregrine concedes the point but notes that many prideful things are done for God. He explains that he originally joined the Church because he wanted to meet Christ in person.
In First Town, Wilder is dealing with administrative matters when Halstead returns his call. He asks the general to apply more discretion when choosing immigrants for Mars. Halstead warns that they are getting more request for immigration than they can handle as things get worse. Wilder then checks with the search party looking for the missionaries, but they haven’t found anything so far.
Stone and Peregrine doze off and the blue lights approach them again. Peregrine wakes up but chooses not to wake up Stone. He believes that they are sentient and tries to find a way to prove it, and then walks to the edge of a nearby cliff. After a moment, Peregrine offers a prayer and then throws himself off. One of the lights catches him and sets him gently on the ground. He tells them that they understand right and wrong, and offers to build a church in the hills with a blue sphere to represent the Martian Christ.
Two of the spheres leave and the third one stays as Peregrine says that he and the others will help them discover God. That one addresses Peregrine by name, saying they are the Old Ones. They had bodies once and learned to free themselves, and left their cities long ago. They will never die and live in the grace of the being that Peregrine calls God. Without bodies, they have left sin behind. It thanks Peregrine for the thought but tells him that each of them are temple until themselves. The Old One tells Peregrine to tend to his own kind because they are at peace, and then leaves with the others.
The next morning, Peregrine goes back and wakes up Stone. When Stone wonders if they will go further into the hills, Peregrine says that he knows the way back. Stone asks what he means and Peregrine tells him what happened. He starts walking back to First Town and Stone asks why the Old Ones left him. Peregrine says that he didn’t have the heart to ask them to take him back, and that one of them spoke to him. He tells Stone that it was His voice and then continues walking.
At the Lustig home, Anna wonders where David went and Leif assures her that their son will soon be back. As she goes back inside, Leif hears David whistling again and sees him walk out of the desert. When David wonders if he’s going to ask him who he is, Leif tells him no and asks where he’s been. David explains that he was near First Town and almost got caught, but doesn’t want to talk about it. Leif agrees and they go in for supper.
As they eat, Anna suggests that they go into town that evening, but David nervously offers to stay at home. His says that he’s afraid of the town and the people there, but Anna refuses to hear otherwise As they take the canal boat into town, David asks Leif to stay close so that he won’t be caught. When they land, David holds Leif’s arm and again insists that he stay close, and Leif promises that they won’t stay long. However, someone bumps into David and he starts to follow them. Leif calls him back and David warns that it’s happening. However, when a crowd passes by, David is drawn to them and walks away. Leif tells Anna to go to the diner while he looks for their son, and to meet them at their boat in an hour.
Leif walks through the town, looking for his son, and runs into Wilder. He tells him that David is missing and insists that something is wrong. When Wilder recognizes the name, he remembers that David disappeared on the second expedition. He begins to suspect something but Leif realizes that he’s said too much, apologizes for disturbing him, and continues his search. As he goes, Leif mentions that his son is 24 and Wilder remembers that David was 24 when he disappeared on the second expedition. He goes after Leif but the older man has disappeared into the crowd.
Father Peregrine is at his office in the small First Town church when he hears someone outside. He goes into the empty chapel and finds the doors open but no one there. Peregrine turns back and genuflects before the cross, and then hears something dripping into the basin. The father goes to investigate and sees blood dripping into the basin. Christ himself is standing behind the basin, blood from his palm wounds dripping into the water. Peregrine kneels before him, and Christ begs the father to let him go. Trembling, he asks Peregrine to avert his gaze because the more he looks, the more he becomes Christ. He explains that he is not Christ and that he came there by mistake. When he lost hold of Leif, he became many things to many people and fled into the church. But when Peregrine came in, he became trapped in his current form.
Peregrine realizes that “Christ” is not what he seems, and Christ begs his forgiveness, saying he cannot be what he appears. The father totters on the brief of insanity and Christ says to release him lest he go with him. Peregrine admits that he can’t, but Christ tells him that he is only seeing his own needs. He tells Peregrine to look away and then he will be gone, but otherwise he can’t hold Peregrine’s needs in his form. Peregrine wonders what Christ is, and it says that the father knows. He explains that when Peregrine came into the church and looked at the crucifix, his dream of meeting Christ seized him again and seized him as well, causing him to bleed. Realizing what he’s done, Peregrine turns away and tells his visitor to leave before he keeps him there forever. When he turns back, Christ is gone
Wilder comes to the church and finds Peregrine kneeling before the altar. The father reminds him of their earlier conversation about Spender and says that he may have been a Martian, and there is a Martian in First Town now who can look like anyone that the viewer has in mind. When Wilder asks how he knows, Peregrine says that he saw him but refuses to clarify further.
Leif continues his search and runs into a friend. The man mentions that a worker, Joe Spaulding, just found his missing daughter Lavinia, who everyone thought drowned in the canal. The friend explains that the Spauldings were walking home near the church when they saw their daughter on the street and took her home. Realizing who it is, Leif runs to the Spaulding home and sees Lavinia in an upstairs room. He throws a rock to get her attention and says that David needs to come home. Lavinia says that there’s nothing Leif can do, but he insists that his mother is waiting and she can’t bear to lose her son a second time. The daughter says that the thoughts in the house are too strong and she can’t change back. Leif insists that they are a family of five and can take her loss better, and refuses to let her go. Lavinia begs him to stop, but Leif promises that she’ll be safer outside of town and they’ll never come to First Town again. The girl starts to come down and Joe hears them and tells Lavinia to stop. Caught between the two mental forces, Lavinia drops to the ground and becomes David, and Leif pulls his son off while Joe comes after them.
Leif and David come to an alleyway and Leif sends his son down it to the boat while he leads off Joe. Wilder finds Leif and warns him that his son isn’t who he thinks, but Leif shoves past him. Meanwhile, David runs by a policeman who sees him as a prisoner that escaped and chases after him.
Leif gets to the boat where Anna is waiting. However, Joe runs up and grabs David, who turns into Lavinia. The policeman also arrives and grabs Lavinia, who becomes his prisoner. As more people arrive, David takes on the form of each person that they want to see most. Wilder tells them to stop, yelling that he’s a Martian. Overpowered by all the thoughts, the Martian collapses to the ground, choking. As Wilder kneels beside him, his face takes on the appearance of all of the desired ones in the crowd and then finally becomes that of a Martian. Dying, he looks up at Wilder and then dissolves into nothing.
Wilder returns to home where Ruth is waiting. She tells him that Earth has signaled that there’s going to be a war. They tune in a broadcast saying that all communication has been shut down and the world is on alert. Ruth wonders what they’re going to do and Wilder tells her that he’s going back to Earth to get his brother Bill and their family and bring them to Mars where it’s safe. When Ruth asks why they can’t wait until the next shuttle, Wilder warns that there won’t be another shuttle. She suggests that they go as a family back to Earth but her husband tells her that Mars may be the only home left to them. When Ruth warns that they’ll be cut off and forgotten, Wilder warns her that there may be no one left to forget them.
Wilder’s call to Bill at Mission Control goes through and Bill says that it’s all over. He tells Wilder that Congress has cut the budget to space exploration to pay for defense, and that they plan to shut down the colony. Wilder insists that he’s bringing him to Mars, explaining that they know enough to support a small group, and tells him to hold on until he can get there.
The next morning, Wilder drives to warn small groups of colonists of the coming evacuation. He drives to a deserted stretch where two highways meet and where Sam Parkhill has set up his diner. Parkhill and his wife Elma are the only ones there, dressed up in gaudy cowboy costumes. They invite him to have a meal and Wilder tells them about the evacuation. Parkhill doesn’t believe it, figures it’s another rumor, and insists that they’re going to stay with the diner in anticipation of all the workers coming by rocket. Wilder tells him that no one is coming but Parkhill doesn’t believe it. Elma smiles weakly and Wilder wishes them well and says that they have until morning to get to town and board an evacuation ship.
Parkhill goes back to the diner counter and sits with his wife, waiting. He promises that they’ll soon have more business than they can handle, but Elma doesn’t believe they can trust the people on Earth to send workers. A masked Martian comes in behind them and Elma sees him first. It addresses Parkhill by name, saying it has come to talk with him about an important matter. He holds up a metal device and Parkhill draws his gun and shoots the Martian, assuming it’s attacking him. It falls and then disappears, its robes and mask collapsing around it. Elma examines the metal device and realizes that it’s not a gun but a message. Parkhill insists that he didn’t know and was just protecting them, and they bury the robes in the desert.
Afterward, Parkhill apologizes to Elma but insists that he thought the Martian was drawing a weapon. Elma looks outside and sees four Martian sandships approaching the diner. Parkhill says that he bought the last one at an auction and there aren’t any more left, and they realize that the Martians are piloting them. Parkhill drags Elma outside but remembers that he never fixed his truck. He tells Elma that they’ll have to escape in the sandship, reminding her that the Martians killed the first two expeditions.
Parkhill manages to start the sandship and he flees into the desert with the Martians in pursuit. As the Martians close in, Parkhill fires at them, killing the crew on two of the ships. The Martians aboard disappear and the sandships go out of control, but Parkhill runs out of ammo. The two remaining sandships cut them off and bring them to a halt. The Martians board their ship and tells Parkhill to put his weapon away. They then give him the same metal message as the first one and explain that it is a grant of territories covering half of Mars. The Martians then tell him that tonight is the night and that Parkhill should prepare. They then leave as they came, disappearing back into the desert.
That night, Elma studies the skies through a telescope while Parkhill admires the grant. He wonders why the Martians didn’t kill him and what they meant by tonight. Parkhill figures that the rockets are coming that night. Elma finds Earth in the sky and Parkhill admires it through the telescope... and then watches a nuclear explosions envelop it. Shocked, Parkhill backs away and Elma checks the view, realizing what has happened. Half shocked, half mocking, she tells her husband to open the doors and fire up the grills, but warns him that it looks like it’s going to be an off-season.