The third Mars expedition from Earth arrives and the crew explores a Martian city. The inhabitants are gone but the buildings are in perfect condition. They discover black ash on the ground and the ship's archaeologist, Spender, confirms that the ash is the remains of the Martians. One of the previous expeditions infected the Martians with chicken pox, and the resulting fever caused the Martians to literally burn up from within. Spender bemoans the casual destruction of an entire civilization and ignores Captain Wilder's objections that they were unaware of what they were doing.
That night, the crew celebrates and asks Wilder when they can go into the city. Spender avoids them until the next morning, when they approach the city. One of the crewmen, Biggs, is still drunk from the previous night's celebrations and starts giving all the landmarks Earth names. Spender finally attacks him and Wilder and the others separate the two men. Wilder talks to Spender in private and tells him that he has to put him on report. Spender explains that he was ashamed at the behavior of his fellow Earthmen, and insists that the Martians are still watching them. He warns Wilder that they will ruin the planet but Wilder disagrees. He tells Spender to learn the Martian names and preserve them.
The crew explores the city and Spender enters what appears to be a museum-library. He finds a Martian mask and shows it to Wilder. The captain believes that the Martians would accept their presence, as children, but Spender disagrees. He finds a book-like object and realizes that it contains the equivalent of millions of books' worth of information. Spender says that he wants to stay. The other crewmen enter and Biggs casually grabs the book and then destroys it.
Three days later, crewmen Cheroke and Parkhill inform Wilder that Spender has disappeared. They are reluctant to keep searching but Wilder orders them to continue. Meanwhile, Biggs is shooting at bottles. Spender comes up behind him, wearing the Martian mask. Spencer says he's the last Martian on the planet and then shoots Biggs dead.
Cheroke, Cramer, and two other men are tending the campfire when Spender approaches them. He tells them that he's been living in the city, reading the Martian books and learning of their culture. They are surprised to hear Spender say that he's found a Martian. The archaeologist is more interested in learning what they would do if they found people destroying their civilization. Cheroke, a Sioux, says that he would sympathize with the Martians because the white men did the same thing to his people. Cramer and the others are unsympathetic. Spender then explains that the Martian asked him for everything he had, and he willingly gave it so the Martian could enter the camp. Spender then draws his gun and kills everyone but Cheroke. The Sioux looks at him in horror as the archaeologist offers him the chance to join him because of his sympathies. Cheroke refuses and Spender reluctantly guns him down.
The next morning, Wilder and the others find the corpses. They start hunting Spender, and Parkhill in particular is eager to kill someone he now views as a traitor. Spender fires back at them but clearly isn't trying hard. They chase the archaeologist into the museum-library and Parkhill is eager to toss in a few grenades. Wilder stops him and goes inside alone. Spender explains that he lost his resolve after killing the others and came back to the museum-library to build his strength back up. The archaeologist explains that he plans to kill the remaining crewmen. When the third expedition mysteriously disappears, the Earth authorities will delay. When they finally do spend further expeditions, Spender will greet them as a sole survivor and then kill them off.
Spender explains to the captain that the Martians learned how to live in harmony with nature rather than question or conquer it. He asks Wilder to join him but the captain refuses, warning that Spender is surrounded. The archaeologist insists that he'll win because he's the better killer now that he has something to fight for. However, he agrees to let Wilder go back to his men and start the attack. Spender notes that while Wilder is like him, he's able to handle it better because he hasn't killed anyone. He promises not to kill Wilder no matter what happens, and says he'll give the captain another chance once he's killed off the rest of the crew. Wilder insists that he'll keep coming after him no matter what and gives Spender a last chance to surrender. Spender refuses and asks him to try and keep the others from destroying the remnant of Martian civilization. Wilder says he'll do what he can. As Wilder returns to the others, Parkhill tries to shoot Spender as he reveals himself. Wilder stops him and orders him and the others out.
As Spender contemplates the Martian mask, Wilder calls out and gives him 30 seconds to run away. Spender refuses and prepares to shoot. Wilder tries to stop him but Spender slowly draws his gun and the captain is forced to defend himself. As Spender lies dying, he tells Wilder that he's Spender now, and gives him the mask. The archaeologist tells Wilder that he has quite a job ahead of him, and then dies. Wilder takes the mask and then contemplatively starts to put it on.