The story starts with the philologist Elwin Ransom, some years after his return from Mars at the end of Out of the Silent Planet, receiving a new mission from Oyarsa, the angelic ruler of Mars. After summoning Lewis, the first person narrator, to his country home, Ransom explains to Lewis that he (Ransom) is to travel to Perelandra (Venus), where he is to counter some kind of attack launched by Earth's Black Archon (Satan).
Ransom meets the King and Queen together with the Oyéresu of Mars and Venus, the latter of whom transfers its divinely ordained dominion over the planet to the King and Queen. All the characters celebrate the prevention of a second biblical "Fall" and the beginning of a utopian paradise in this new world. The story climaxes with Ransom's vision of the essential truth of life in the Solar System, and possibly of the nature of God: strongly paralleling the journeys of Dante in the Divine Comedy.
His mission on Venus accomplished, he returns, rather reluctantly, to Earth in the same manner in which he made the outward journey; his new mission is to continue the fight against the forces of evil on their own territory.