When you run out of sci-fi stories to tell why not just retell a familiar story and set it in space? Plus the film was scientifically accurate – it says so on the poster!
Commander Christopher “Kit” Draper (Paul Mantee) and Colonel Dan McReady (Adam West) reach the Red Planet in their spaceship, Mars Gravity Probe 1. They are forced to use up their remaining fuel in order to avoid an imminent collision with a large orbiting meteoroid; they descend in their one-man lifeboat pods, becoming the first humans on Mars.
Draper eventually finds a rock face cave for shelter. He figures out how to obtain the rest of what he needs to survive: he burns some coal-like rocks for warmth after discovering that heating them releases trapped oxygen. This allows him to refill his air tanks with a hand pump and to move around in the thin Martian atmosphere. On one of his excursions, he finds McReady’s crashed pod and dead body. He then finds their flight-test monkey, Mona, alive and returns with her to the cave. Draper then constructs a crude sand alarm clock to awaken him for periodic doses of oxygen.
Later, he notices that Mona keeps disappearing and is uninterested in their dwindling supply of food and water. He gives her a salty cracker but no water. When Mona gets very thirsty, he lets her out and follows her to a cave and an underground pool, which also has edible plant “sausages” growing in the water.
As the days grow into months, Draper slowly begins to crack from the prolonged isolation. He watches helplessly as his mothership, an inaccessible “supermarket”, periodically orbits overhead; without fuel, the spaceship cannot respond to his radioed order to land.
While walking about, Draper comes upon a dark rock slab standing almost upright. Curious, he digs in the ground around it, exposing a skeletal hand and arm wearing a black bracelet. He uncovers the rest of the humanoid skeleton and determines that the alien was murdered; the front of the skull shows heavy charring. To hide his presence on Mars, Draper signals his low-orbiting mothership to self-destruct on its next overhead pass.
Just in time, as Draper sees a spaceship descend and land just over the horizon. Believing it might be a rescue ship from Earth, the following morning he heads towards the landing site, only to see alien spacecraft in the sky. He approaches cautiously and sees human-looking slaves being used for mining by equally human-shaped captors wearing spacesuits and holding weapons on their bare-skinned captives. One of the slaves (Victor Lundin) escapes, running into Draper; an alien ship blasts their area as the two escape. Draper notices the stranger is wearing black bracelets just like the one he found in the grave. The aliens bombard the mine area that night and then depart. When he and the stranger investigate, they find the bodies of the other slaves.
Draper names his new acquaintance “Friday,” after the character in Robinson Crusoe, and begins teaching him English. In return Friday shares his “air pills”, which provide oxygen; they gradually grow to trust and then like each other. After a while, the alien spacecraft returns, tracking Friday by his bracelets. Draper begins sawing away at the tough material with a wire hacksaw. When the aliens blast the castaways’ hiding place, Draper, Friday, and Mona flee north through the underground Martian canals. They eventually surface near the polar icecap. Exhausted, freezing, and nearly out of the air pills, they build a snow shelter. Draper finally succeeds in cutting off Friday’s bracelets shortly before a meteor crashes into the ice cap; the resulting explosion and firestorm melts the ice and snow, saving them from freezing to death.
Later, Draper detects an approaching spaceship. He fears it is the returning aliens, but is relieved when his portable radio picks up an English-speaking voice. Draper and Friday watch a descending rescue capsule; Mars recedes in the distance as the film credits scroll.