The nations of Earth unite to begin the exploration of Mars, sending an unmanned probe. Its sensors extend to the horizon and it detects nothing. However, Mission Control is unaware that a Martian city lies just over the next ridge.
January 1999 - The First Expedition
The first manned expedition to Mars is prepared and Colonel John Wilder and Captain Burt Conover meet with the press. Wilder tells them that Captain Nathaniel York, Conover's fellow pilot, will be along shortly. When the reporters wonder if the controversial Jeff Spender will be going, Wilder tells them that Spender will help in Mission Control. He then turns the briefing over to Conover and goes outside, where Spender is waiting with York. Spender is unhappy that he wasn't assigned to the first expedition and Wilder assures him that he'll be on the next one. As York goes in to join Conover, Spender assures Wilder that he won't spoil the moment.
After the briefing, Wilder goes to the control room and chats with his brother, Captain Bill Wilder. He reminds his brother about their family dinner the next Sunday and then escorts Conover and York to the departure chamber where everyone gives them a round of applause before they board Zeus I.
On Mars, Mr. K and his wife Ylla live alone by the Fossil Ocean. However, Mr. K overhears his wife muttering in his sleep and goes to listen. She mentions the name "York" and then wakes up, and he asks what she dreamt of. Ylla explains that the man she dreamed of, York, said that he came from the third planet, Earth, but Mr. K reminds her that their scientists have determined that the air there is too thick to sustain life. As he walks away, Ylla considers her dreams, fascinated.
York and Conover arrive in Mars orbit and shift to the far side, losing contact with Mission Control.
Ylla has another dream of York and Conover landing on the planet and meeting her. When she wakes up, she discovers that Mr. K has been listening again. Ylla says that York called her beautiful but dismisses the whole thing as nothing but a dream. However, her husband demands that she tell him everything, and Ylla says that York promised to take her back to Earth with him. After interrogating her, Mr. K abruptly agrees that it is merely a dream even though she has described everything in detail, even when the lander will touch down in Green Valley.
That night, Ylla look up at the sky and then asks her husband if he's is going into town as he always does on that day. Mr. K says that he has other matters to attend to and Ylla says she's going to see a friend. Her husband points out that the friend lives in Green Valley and then tells Ylla that she can't go because he has a mission to perform. When she agrees to stay, Mr. K arms himself and she realizes what he plans to do. He insists that he is justified in killing to protect their world, dons the Mask of Conflict, and leaves.
As she waits, Ylla hears the lander arrive... and then gunshots. After a few minutes, Mr. K returns alone and says nothing of what happened.
In Green Valley, York and Conover lie dead and the sand wears away at their lander.
Back on Earth, General Halstead considers shutting down the program, but Wilder disagrees. He warns that their superiors will insist on another expedition because they need to determine if Mars can be colonized. Halstead admits that he's right and that the pressure is on for colonization of the red planet. Wilder wants to lead the second expedition but Halstead refuses, saying that he's too important to risk. As Wilder leaves, Spender confronts him and asks to be on the second expedition if he can. Well aware of the plans to colonize if the planet will support life, Spender wonders if they have the right. Wilder doesn't see the problem and points out that they don't even know if there is any life there to despoil.
April 2000 - The Second Expedition
Captain Arthur Block and his crewmates David Lustig and Sam Hinston emerge from the Zeus II rocket and find themselves facing an early 20th century American town. The three men try to work out a theory that could account for the town's existence on Mars. The trees are over a century old, and Conover and York couldn't have built in an entire town in that period even if they had resources. Hinston suggests that a group during World War I somehow traveled from Earth to Mars, but Black points out that no one could have kept such a project secret all that time. They couldn't have somehow passed through a time warp and returned to Earth because the atmosphere is the thinner air of Mars. Finally, Black recognizes the town: it's Green Bluff, IL, the town that he grew up in.
The men go to a house and knock at the door. A woman answers and tells them that the year is 1979 and they're in Green Bluff. When Black points out that she's on Mars, the woman ignores her and goes back inside. The crewmen wonder if the people did come from Earth and are suffering some kind of psychosis. Before they can pursue the matter, Black sees a familiar face nearby: his younger brother, Edward Black. As Black runs to embrace his brother, he notes that his brother died when he was 19. Edward tells him that their mother and father are there despite the fact they died as well, and Black wonders if Mars is Heaven. His brother simply says that it's a place where they can have a second chance and tells Black not to question it. As the brothers race each other to the house, Hinston and Lustig try to call their captain back. However, Lustig's grandmother and Hinston's Aunt Thelma come over and soon take them away to their own homes.
When Black and Edward get to their home, Edward wins the race and Black complains about the thin Martian air. They go inside and find their mother playing the piano. Black goes to her and breaks into tears as he hugs her. Mrs. Black assures him that everything is all right as his father comes in.
That night, the Blacks celebrate with dinner and then the captain dances with his mother. They tell him just to accept what is happening and be happy, but Black wants to go back to the ship to check in. Mrs. Black tells him that if he does, he'll miss the surprise: Marilyn Becker, his old girlfriend, coming in from Chicago. They assure him that he can check on the ship in the morning and Black goes outside to meet Marilyn. She explains that she told her parents that she doesn't care if they cut off her allowance as long as she's with him, and asks if he's sorry that he's there. The captain assures her that he isn't, but is surprised that her father hasn't had him locked up. Black tells her that he just needed a little encouragement from her to pursue her, and Marilyn says that he's getting it now as she kisses him.
Later, Black and his brother go to their old bedroom to sleep for the night. As the captain starts to doze off, he gets an idea and Edward asks what it is. Black says that he's wondering if the town and the people there aren't real, and that they're actually disguised Martians. Edward points out that would mean that he isn't really Edward, and then admits that he isn't. He walks calmly toward Black and explains that when they saw the rocket coming, they defended themselves using the only weapon they had: telepathy. They scanned Black's mind and took the memory of his hometown, and then telepathically implanted the image of the town and took the form of people that he and the others knew. Black starts to feel dizzy and the Martian tells him that they needed to divide up the three men.
Black tries to get up but collapses, and the Martian tells him that they poisoned his dinner earlier with a painless, fast-acting poison. As Black starts to die, he asks why they're murdering him. The Martian tells him that they saw the violence of earthmen in his mind and knew that they would eventually come to Mars. As the Martian takes on his true form, he apologizes for murdering out of fear and asks the dying Black to forgive them.
The next day, the Martians are still under the influence of the stolen memories and conduct a burial service near Zeus II. As the memories fade, they walk away and the town vanishes.
On Earth, the two Wilder brothers take their wives to a restaurant and Bill asks Wilder to reconsider going on the third expedition. He insists on going and they're interrupted when Wilder's wife Ruth comes over with Bill's wife. Spender and Sam Parkhill, two of the crew going with Wilder, arrive at the restaurant and Spender assures their commanding officer that everything checks out. Parkhill discusses the trip to Mars with Bill and his wife and says that he plans to take his wife Elma there once they begin colonization and start a diner when he retires.
Outside, Ruth talks to Spender, who is arguing against colonization again. Wilder's wife doesn't believe that they would destroy the civilization there, but Spender disagrees. They all share a toast to a safe journey and leave. Back at home, Ruth sits on the patio and Wilder comes out to reassure her. She worries that the third expedition will meet the fate of the first two, but her husband promises her that they won't fail.
June 2001 - The Third Expedition
Zeus III lands on Mars near a Martian city and Spender flies in to investigate. One of the other men, Briggs, wants wilder to break out the liquor ration to celebrate but Wilder tells him to hold off. Spender finally returns and tells them that while four out of five cities have been dead for thousands of years, the fifth is always occupied by recently dead Martians. All of them appear to have died of chicken pox, and Spender figures that someone brought it on one of the first two expeditions. A few may have survived, but the Martians have no possibility of repopulating. Briggs laughs at the triviality of it until Wilder tells him to shut up. Briggs tells Spender to relax but the man brushes him off and goes into the rocket.
Later, the crew breaks out the liquor and Briggs gets drunk. As he prods the other crewmen Cookie and McClure into dancing, Spender watches them from his cabin window. He finally goes down when Briggs takes garbage to the nearby canal and punches him. Wilder and Parkhill investigate the noise and Parkhill takes Briggs back to the camp while Wilder demands an explanation. Spender admits that he was ashamed for all of them for their disrespect toward the Martians, and that they're responsible for the Martian extermination. Wilder disagrees, but Spender figures that their superiors would have authorized the expeditions even if they had known the Martians were vulnerable to human diseases. He figures that the Martians are watching them because the old always know when the new arrive, and warns Wilder that when earthmen come in force, they will apart the planet. Wilder refuses to believe it, but Spender say that if there are any surviving Martians then they'll hate the earthmen who have come. The colonel disagrees, saying that they would think of them as children and welcome them into their cities. He believes that earthmen will eventually learn but Spender remains unconvinced. With that, Wilder tells spender that he won't report him but he doesn't want a similar incident to occur.
The crew goes into the nearby city to explore. A hung-over Briggs finally puke on the street and a disgusted Spender runs off, telling Wilder that he'll be back. He doesn't come back by nightfall and Wilder figures that he's hiding, but Parkhill thinks that their crewmate looked different after he entered the city.
A week later, spender returns to the ship and find Briggs washing his feet in the nearby canal. When Briggs asks where he's been, spender tells him that he's the last Martian and shoots him dead with a Martian gun. Parkhill and Wilder are out exploring, and Spender goes to the camp and confronts McClure and Cookie. When they ask where he's been, Spender tells them that he spent the week in the city learning the Martian language and reading their books. He tells them that a Martian followed him back to the camp. When they say that they don't see a Martian, Spender shoots them both dead and walks back into the desert. Wilder and Parkhill return a short time later and find the three corpses. Realizing that Spender is responsible, they grab guns and head out after the man.
Spender is in the hills and sees the two men coming. He opens fire on them and easily avoids their attempts to surround him. However, Spender offers them a truce and invites just Wilder to come see him. Wilder agrees against Parkhill's advice and finds Spender waiting for him. He tells Wilder that Earth will eventually send thousands of colonists, contaminate the planet, and eventually blow it up just like they're in the process of blowing up Earth. Spender figures that once he kills Parkhill and Wilder, he can wait and greet the next expedition. Once he kills them, he'll wait for the next expedition, and the next, and hold them off until he dies. When Wilder points out that they outnumber him, Spender tells him that he's found underground tunnels that will let him escape and then come back when they're off-guard and kill them.
Wilder can't believe that Spender will kill them, and Spender asks for a half hour to show him what the Martians once had. The colonel agrees and they go into the city where Spender explains that the Martians learned how to accept life as it is and live with nature. When Wilder notes that the Martian skilled the first two expeditions, Spender defends it as self-preservation. He invites Wilder to join him, saying he's not like the others, and says that there are still a few Martians left. Wilder reluctantly refuse and Spender lets him go back. However, the man asks Wilder to promise that he'll do what he can to stop the earthmen from ripping apart the planet and what's left of the Martian civilization. Wilder says that he'll do what he can and Spender tells him to turn around. Wilder reluctantly does so and when he turns back, he discovers that Spender has slipped away.
Wilder goes back to Parkhill and tells him that their former crewmate won't surrender, and that he's to kill Spender with a clean shot. The two men move into the city and a masked Martian opens fire on them. The figure uses the tunnels to avoid them briefly, but Wilder manages to mortally wound their attacker. When they remove the mask, they discover that it's Spender. As he looks at his friend, Wilder realizes that Spender was right and that Man will come to Mars no matter what.