The narrator, Lewis, goes to meet his colleague and friend Ransom. On the way he is plagued by doubts and fears which are sent by the fallen eldila of Tellus (Thulcandra), but he eventually arrives. In Ransom's home, he comes sees the Oyarsa of Malacandra who is there to send Ransom to Venus (Perelandra). The narrator helps prepare Ransom's coffin-like transportation and he is sent off. Over a year later, the narrator returns to Ransom's home to help free him from his return flight. Ransom looks extremely healthy except for a wound on his heel that won't stop bleeding, and he appears 10 years younger. Ransom then relates the tale to Lewis.

On Perelandra, Ransom lands in a giant ocean. Eventually he finds his way to a floating island, covered in beautiful plants. He eats from a gourd and it is so pleasurable he is surprised not to feel any guilt. Soon, he meets a green woman and they begin conversing. She and the King are the only humans on the planet. They are human because, ever since the son of Maleldil took on human form on Thulcandra, all hnau (sentient beings) in the universe will now be human. Ransom learns that the Queen and King can walk on Fixed Land, but they cannot sleep there. The Queen and Ransom see a ship crash into the ocean. Weston has arrived on Perelandra.

Weston, possessed by an evil demon (possibly even Satan) begins trying to convince the Queen that she must stay on Fixed Land in order to show Maleldil that she has grown beyond a mere child. She resists the temptation, with Ransom's help, but it is clear that she is beginning to wear down. Ransom, realizing it cannot go on, physically attacks Weston (or the Un-Man as he calls him now) in order to destroy his body and save the Queen. They fight brutally, escape on fish to the Fixed Land, and emerge in an underground cave. Ransom strangles the Un-Man.

Ransom spends the next few hours searching for an escape from the cave, eventually finding his way to a huge cavern full of fire. At this point the Un-Man returns, Ransom smashes his head in with a rock, and then throws the body into the fire. He finally escapes the cave to a mountain top where he is nursed back to health by Perelandra. When he recovers, he makes a long hike to the top of a nearby mountain, where he finds the Oyarsa of Malacandra and Perelandra. The King and Queen are on their way to inherit the planet from the Oyarsa.

When the King and Queen arrive, they thank Ransom for his help, and Ransom gets a glimpse of the true potential of humans on earth, had it not been for their own failure to resist Satan's temptation. A year passes on the mountaintop during which they praise Maleldil. Ransom is returned to Earth.