The novel starts with Dr. Elwin Ransom walking through the English countryside during a year off from his work as a professor of language at Cambridge University. Ransom looks for a place to stay for the night, eventually coming to a large estate. The gate is locked, but Ransom hears a commotion and sneaks in through a hedge. He sees two men, Dr. Weston and Mr. Devine, struggling to capture a young boy. Ransom convinces Weston and Devine to let the boy go home and goes into Weston’s house for a drink. The drink turns out to be drugged, and Ransom has a strange dream of meeting aliens while under its influence. When he wakes, Ransom finds himself in a spaceship. He overhears Weston and Devine say that they have kidnapped him to be a sacrifice to a mysterious people called the sorns on a planet called Malacandra.
Ransom tries to worry about his fate on Malacandra, but can’t help but spend the journey in awe of the bright vitality of the heavens – he can’t bring himself to call this expanse “space” now that he has seen how beautiful it is. After a month’s flight, the ship begins its descent to the planet of Malacandra, and Ransom’s fear returns despite the stunning landscape of Malacandra. They land at a settlement site on the shore of a gorgeous lake. As they unpack their supplies, Ransom sees three long, ghostly figures walk across the lake. He assumes these must be the sorns, and runs from Weston and Devine in terror. He flees through the alien forest behind the settlement.
Ransom rests for the night, then continues to walk on the next morning. He stops at a pond to take a drink, and then sees a large, seal-like, black creature also drinking from the pond. Ransom is again overcome by fear until he hears the creature make noises that seem like speech. Ransom decides this creature must be intelligent and goes to meet it. The creature introduces itself as a “hross” named Hyoi and takes Ransom back to his village. Ransom lives among the hrossa for weeks, learning their language and finding out about their peaceful culture from Hyoi. The hrossa are experts with boats and love to create poetry and songs, making Ransom reassess his judgement of the hrossa as a primitive society. Ransom is especially struck by the lack of any conflict between the hrossa, the sorns, and another species called the pfifltriggi, as well as the hrossa’s happy acceptance of death as the natural end of a life well lived. Hyoi also gives Ransom an introduction to hross religion, which includes spirits called “eldila” who due the bidding of the head eldil, Oyarsa. Oyarsa is the mouthpiece of the ultimate gods of the universe, known as the Old One and Maleldil the Young.
Many weeks after Ransom’s arrival, the hrossa become excited about news of a shark-like animal called a hnakra in the lake near their village. All the hrossa ready their boats to hunt the hrossa, and Ransom prepares to ride with another hross in Hyoi’s boat. Their small party embarks with the rest of the hrossa to search for the hnakra, the only sign of evil in this otherwise idyllic place. An eldil then finds their boat and informs Ransom that Oyarsa would like to see him, but Ransom puts off this order so that he and Hyoi can continue to hunt. After a morning of sailing to the lake, Hyoi and Ransom sight the hnakra and manage to kill it, though wrecking their boat in the process. On the shore, Hyoi and Ransom celebrate their victory, but their triumph is cut short when Weston fires a gun from the nearby forest and hits Hyoi in the chest, killing him. Deeply grieved by Hyoi’s death, Ransom decides to follow Oyarsa’s orders and go to Oyarsa’s home in Meldilorn.
Ransom gets directions to Meldilorn from another hross and sets off on his journey. He must climb out of the forested area of Malacandra, the handramit, and scale the mountains onto the highlands of the planet, the harandra. Ransom travels for a day, but quickly finds that there is less atmosphere on the harandra and begins to suffocate. He makes it to the home of a sorn named Augray who gives Ransom an oxygen mask so he can survive. After spending the night at Augray’s cavern and learning that the sorns are more scientifically minded but just as kind as the hross, Ransom continues on to Meldilorn. He reaches this sacred island, going down into another handramit even more beautiful than the last one. A hross meets Ransom on the island and shows him stones with scenes of the history of Malacandra. Through these scenes, Ransom learns that Malacandra is Mars, and that Earth also once had an oyarsa, but Earth’s oyarsa is now evil.
Oyarsa calls Ransom and Ransom appears before him, seeing Oyarsa as an indescribable figure of light. Oyarsa tells Ransom that Earth is known as Thulcandra (meaning the silent planet) because Earth’s Oyarsa turned against the Old One and Maleldil and cut Earth off from the rest of the heavens. This ancient battle explains humanity’s “bent” nature, as Earth’s oyarsa, now known as the Bent One, convinces humans to care only for themselves. Ransom’s meeting with Oyarsa is interrupted when a group of hrossa bring in Weston and Devine to stand trial for killing three hrossa (including Hyoi). Weston refuses to respect Oyarsa, believing that all Malacandrians are savages who believe in pagan nonsense, while seeing it as his duty to colonize Mars for the survival of the human race. Devine, for his part, only cares about the gold on Malacandra. Oyarsa sentences both Weston and Devine to leave Malacandra forever, but gives Ransom the choice to stay. Ransom decides to go with his fellow humans and bring news of the paradise of Malacandra and the will of Maleldil back to Earth.
Ransom, Weston, and Devine are given oxygen and food for their spaceship, but the current orbital paths of Mars and Earth mean they must fly dangerously close to the sun to get home. Somehow, they make back to Earth, but Ransom is too afraid of Weston to share his story. It is only when Lewis, a former student of Ransom’s, asks a question about a mention of “oyarsa” in an ancient Latin text that Ransom shares his experience. Ransom and Lewis decide to publish Ransom’s adventure as a fictional story, so that the world will not reject them as lunatics, and so their readers can consider the value of the Malacandrian way of life and perhaps implement some of those ideals on Earth.