An unmanned probe lands on Mars. It detects no signs of life and sends its readings back to Earth. However, the probe can only scan to the horizon... and a Martian civilization lies just beyond the next ridge.
The first manned expedition to Mars prepares for departure and the unmanned section lifts off successfully. As it moves into orbit, Colonel John Wilder and Burt Conover meet with the press. The reporters wonder if they expect to find life on Mars, and Wilder notes they won’t know until they get there. Wilder assures them that Captain York will be along shortly, and explains that the NATO Alliance plans to launch probes to all the planets in the system. Another reporter asks about Major Jeff Spender, and Wilder informs him that they’ve decided that Spender will remain at Ground Control during the first expedition. Wilder then lets Conover take over to explain the scientific side of things.
Spender is outside with York, and notes that he was just informed that he wouldn’t be on the first expedition. York assures him that he’ll on the next expedition and Spender says that he’d better. They enter the briefing room as Conover talks to the reporters about the ability of Mars to support life. He then introduces York while Wilder asks how Spender is doing. Spender assures him that he won’t spoil the moment.
Later, Wilder returns to the control room and reminds his younger brother, Captain Bill Wilder, to meet with Wilder and his wife for lunch that Sunday. He then goes to see York and Conover off as they take the Zeus 1 manned lifter to link up with the spacecraft in orbit.
On Mars, Mr. K and his wife Ylla live quietly together. While Mr. K listens to his book, Ylla dreams of a York coming from Earth. Her husband hears her call out and takes her hand, and she mutters the name “York.” Ylla awakens and apologizes, and Mr. K asks what she encountered. She explains that she dreamed of a man from another world who came down from the sky and spoke to her of the third planet, Earth. Ylla wonders if it’s possible that the third planet is inhabited and Mr. K reminds her that their scientists have concluded that there is too much oxygen there to sustain life. As he walks away, Ylla finds the thought of people on Earth fascinating.
York and Conover take a landing craft down to Mars and lose contact with Mission Control as they move around the backside of the planet.
Ylla dreams of the lander approaching and the two crewman stepping onto the planet’s surface. She smiles at the thought of York and then awakens to find Mr. K studying her intently. He asks if it was the same dream and she tells him what she saw. Ylla mentions that York described her as beautiful but dismisses it as a dream. Her husband isn’t convinced and asks if there was more, and Ylla says that York offered to take her back to Earth, killing for her if he had to. Mr. K tells her that he overheard her describe everything, even the time it would land, but then agrees with her that it was only a dream.
The lander prepares to touch down.
Ylla looks out at the night sky and Mr. K comments on her sudden interest. She points out that he usually goes into town but her husband says that he has other matters to attend to. Ylla then says that she is going to see a friend, Tria. Mr. K notes that the friend lives in Green Valley and tells her that she can’t go because he has an important mission to perform. Ylla agrees to stay and Mr. K picks up a gun and loads it with bee projectiles. She asks him if there isn’t another way, but Mr. K says that he is justified in killing to protect his world. He dons the Mask of Conflict and tells Ylla to stay there, and then goes out to Green Valley.
As Ylla waits, she hears the sound of the lander touching down. She then hears several gunshots and waits. Mr. K returns alone, but Ylla can only think of York and another world.
On the Mars desert, York and Conover lie dead. Mission Control tries to contact them without success. Later, Wilder’s superior General Halstead considers a discontinuance, but Wilder disagrees. He says it could be any number of factors and points out that they’ll probably be ordered to send another expedition to determine if Mars can be colonized. Halstead admits that it’s likely, and admits that the way things are going on Earth, it may be necessary. Wilder wants to lead the second expedition, but Halstead says that his importance as Project Director rules out any choice.
Outside, Spender asks about the second expedition and Wilder says that he’ll try to get him on it. However, he fears that there may be a second expedition. Spender agrees, but says that if it does occur then he wants to be on it. He wonders that if there is life on Mars, do they have the right to colonize. Wilder doesn’t see the problem with colonization, and wonders if there may even be life as they know it.
The second expedition lands, and Captain Arthur Black, Sam Hinston, and David Lustig emerge from the Zeus II capsule. The first thing they see is an early 20th century town a hundred feet way. Black tries to work out how two planets could evolve the same way. They wonder if Conover and York may have built the town, but Black points out that they couldn’t have done it so soon and created trees a century old. He explains that he was born in Green Bluff, IL, and the town they’re seeing looks almost exactly like it. Lustig suggests that some theory of parallel evolution may be responsible, but Black isn’t convinced.
The three men enter the seemingly empty town and hear birds in the distance and children playing. Hinston suggests that some group may have secretly launched a flight to Mars after the first World War, but Black points out that no one could have kept such a project secret. They go to a house and Black knocks at the door. A woman comes out, speaking English, and asks if they’re salesman. She tells them that they’re in Green Bluff, IL, in the year 1979. Black tells her that she’s on Mars, but the woman disagrees and goes back inside, saying she has things to do.
Lustig wonders if they went through a space/time warp and ended up on Earth, but Hinston points out that the atmosphere matches that of Mars. He sticks to his theory that someone developed time travel in the 1930s and the people developed a psychosis so they believed they were always on Earth. Black disagrees, saying that he knows the town, and then sees his brother Edward Black standing nearby. The brothers run to embrace and Black says that Edward died when he was 19. Edward tells his brother that their mom and dead are alive again and in the town, and that he should just accept it. Black wonders if Mars is Heaven, but Edward simply says that they have a second chance and they shouldn’t question it. The brothers run off and Hinston and Lustig call after them. However, Lustig’s grandmother calls to him and he goes off with her. Hinston’s Aunt Thelma calls him over and he goes off with her.
Black and his brother arrive at their home and Edward wins as his brother complains about the thin air. He hears the piano playing and goes inside. His mother is playing the piano and Black covers her eyes, surprising her. Breaking into tears, the captain hugs his mother and she tells him that it’s all right now that he’s home as his father comes in.
On Earth, Mission Control hasn’t received any transmissions from Black’s expedition in 12 hours.
The Blacks settle down to cake. Afterward, they put on a record and Black dances with his mother. He figures that it’s all a dream but they tell him to accept and be happy. Once the record ends, Black says that he has to go back to the ship but his mother says that if he does then he’ll miss his surprise: his old girlfriend, Marilyn Becker. He remembers that they sent her away so she’d forget about him, and his family insists that he can check the ship in the morning. Black finally gives in and goes outside to wait.
Marilyn arrives and Black wonders how she can be there. She tells him that she told her parents that she simply didn’t care if they cut off her allowance. Marilyn asks if Black is sorry that he’s there and he assures her that he isn’t, but he’s surprised that her father didn’t have him locked up. When he points out that all he needed was a little encouragement, Marilyn tells him that he’s getting it now and kisses him.
Later, Black goes to his childhood bedroom that he shared with Edward and dresses for bed. However, as Black settles down and tries to sleep, he gets an idea. He wonders if they actually landed on Mars and the town isn’t real… and the people aren’t his friends and family. Black suggests that they’re Martians disguised as earth people. When Edward points out that would mean he isn’t Black’s brother, Black turns to stare at him and Edward says that he isn’t. He continues, explaining that the Martians saw the rocket coming and used the only defense they had: telepathy. As Black starts to feel faint, Edward says that the Martian scanned the captain’s childhood memory of his hometown. As Black starts to feel dizzy, Edward explains that the only way to divide and overcome invaders was to use the memories of their dead friends and family.
Black falls out of the bed and collapses, and “Edward” explains that they poisoned his food earlier. He assures Black that his death will be painless, and Black asks why. The Martian tells him that they have seen the violence of him and his kind in his mind, and how they seek out solutions on other worlds rather than find them on Earth. Taking on its true form, the Martian apologizes for murdering out of fear and asks Black to forgive them as he dies.
The next day, the Martians are still under the influence of the crew’s memories and bury the three crew members near their rocket. They then walk away as the illusion of the town slowly fades, leaving the true Martian landscape.
On Earth, Wilder and his brother meet at a restaurant and Bill asks if Wilder won’t reconsider going on the third expedition. Their wives come over and Wilder's wife Ruth insists that they stop talking about business. Wilder’s fellow crew, Spender and Sam Parkhill, arrive for drinks. Spender assures Wilder that he’s checked out everything, while Parkhill tells Bill’s wife that someday they will all be going to Mars. Bill admits that he doesn’t have his brother’s pilot instincts, but Parkhill says that Mars will soon be a family scene and he plans to take his wife Elma there.
Ruth talks to Spender, who objects to Earth colonizing Mars. Ruth doesn’t believe that they would be conquerors, but Spender isn’t convinced. As he goes to get another drink, Ruth assures Wilder that she’s doing fine. Meanwhile, Parkhill talks about how he plans to open a restaurant on Mars once he finishes his tour of duty. Before they start, Bill wishes the three men a safe journey and they all share a toast.
Back at home, Ruth looks up at the sky. Wilder comes out to check on her and assures her that everything will be okay. She doesn’t want to ask him to stay, but wonders why they have to spend a third expedition after the disappearance of the first two. Wilder promises her that they won’t fail this time.
Zeus III lands on Mars near a seemingly deserted city. Spender goes in to investigate while Briggs suggests they break out the liquor ration. Wilder tells them to wait and they wonder what may have happened to the Martians and the other expeditions. Parkhill talks about his dream of building a restaurant again, and they wait until Spender finally flies back. He tells them that the nearby city has been dead for a thousand cities, and three others as well. However, another city 70 miles away was inhabited until a week ago. Now the inhabitants are dead... of chicken pox. Spender figures that someone on one of the first two expeditions infected the Martians, wiping out almost all of them. Even if any survived, they’d have no chance of repopulating. Briggs laughs at the thought until Wilder tells him that’s enough. When Briggs tells Spender to relax, Spender brushes him off and goes to get some food.
Later, Wilder breaks out the liquor ration. Briggs drinks and talks about his sexual escapades back on Earth, and then breaks out an accordion. As he encourages the other crew members, McClure and Cookie, Spender watches them with disgust from within the spaceship. He looks briefly at the nearby city and then tries to go back to work. The drunken Briggs collects the garbage and takes it to the nearby canal to dump, and Spender goes after him. He punches Briggs, knocking him into the canal, and Wilder and Parkhill run up and pull him out. Parkhill takes Briggs to the ship while Wilder demands an explanation. Spender explains that he was ashamed for Briggs and the others, and insists that they have no respect for the Martians. He tells Wilder that they’re responsible and would have sent the expeditions even if they had known that chicken pox would have proven fatal. Spender comments that Martians are still watching them, saying that the old always knows when the new arrives. He figures that they’ll be unable to understand the Martian technology and eventually rip it up and ruin it like they did on Earth. Wilder doesn’t believe it, but Spender warns that if there are any Martians still in the hills, they’ll hate them. The colonel says that there is no hate there and the Martians were a philosophical people that wouldn’t mind them being there. Wilder believes that they’ll learn, but Spender isn’t so sure. He tells Spender that he won’t report what happen but tells him not to let it happen again.
The crew goes into the city and the hungover Briggs continues to belch and whistle for anyone to come out. Wilder tells him to shut up but he stumbles off to vomit. Disgusted, Spender runs off, yelling that he’ll be back. That night, there’s no sign of Spender and Wilder figures that he’s hiding. Parkhill figures that he won’t be coming back anytime soon because he looked different in the city, different than he ever had before.
A week later, Briggs is at the canal kicking his feet in the water and Spender comes up behind him. When Briggs asks where he’s been, Spender tells him that he’s the last Martian and kills him with a Martian weapon. He then goes to the camp and tells McClure and Cookie that he spent a week in a city learning the Martian language and art. Spender then tells them that he met a Martian and followed him back to the camp, and the Martian is there now. They don’t know what he is talking about, and Spender shoots them down and goes back out into the desert.
When Wilder and Parkhill return, they find the three dead man and confirm that they were killed by a Martian weapon. Parkhill figures that Spender has gone insane and they arm themselves to hunt down Spender.
Spender is in the hills admiring a Martian artifact when he sees Wilder and Parkhill closing in on him. He opens fire on them with the Martian weapon. Wilder tells Parkhill to circle around and they try to flank Spender. However, he gets the drop on them but offers a truce. Spender lowers his weapon and invites Wilder up. Parkhill advises against it but Wilder goes up and finds Spender quietly sitting, waiting for him. He tells Wilder that he’s concerned what they will bring to contaminate Mars, and how they will eventually blow it up just like they’re blowing up Earth. Spender figures that if they return then they’ll bring thousands of colonists. If he kills Wilder and Parkhill, then he can live for another 60 years and destroy every expedition that arrives. Spender hopes that the war will finish off mankind or they’ll give up in their efforts to colonize Mars. Wilder tells him that he’ll be dead within an hour, but Spender say that he’s found underground passageways that he can hide until they’re off-guard, and then pick them off.
Wilder can’t believe that his friend has become a murderer, and Spender asks him to come with him for a half hour to show him what the Martians had. The colonel calls down to Parkhill that he’s going with Spender, who leads him into a nearby city. Spender shows him a structure and explains that the Martians learned how to live with nature and to enjoy pure being. Wilder isn’t convinced, pointing out that the Martians wiped out the first two expeditions, but Spender says that they did it just to defend their life blending religion, art, and science. He invites Wilder to stay with him and learn the Martian ways. The colonel considers it and Spender tells him that there are still a few Martians left. He offers to show them to Wilder if he stays, but Wilder says that he can’t. Spender tells him to head back, but asks him that if he wins, he will try to stop the Earthlings from ripping the planet apart. Wilder says that he’ll do what he can, and Spender aims his gun at him and tells him to turn around. The colonel has no choice but to do so and Spender tells him to walk. After a moment, Wilder turns back and discovers that Spender is gone.
Wilder goes back to Parkhill and tells him that Spender won’t surrender. He tells Parkhill to make it a clean shot and they go up into the hills. A figure in Martian robes and a mask waits in the city, and the two men move in carefully. The figure opens fire on them but Wilder finally manages to shoot it. They go to the figure and remove the mask, and discover that it’s Spender. With that, Wilder realizes that Man will come to Mars,