Spoiler alert: This post is part of an in-depth discussion of Perelandra by C. S. Lewis, which will inevitably reveal key plot points. If you don't want me ruining the experience for you, put this post down and go read Perelandra now. I promise I won't be offended.
The story so far: Our protagonist, Elwin Ransom, was brought to the planet Perelandra (Venus) by the Oyarsa of Malacandra (Mars), whom he encountered in the previous book, Out of the Silent Planet. His mission to Perelandra was, as yet, unknown.
Perelandra was a warm, overcast, ocean planet with next to no solid ground. What passed for land were floating mats of vegetation, where plants grew for food and friendly wildlife dwelt. Ransom also met a green human woman, who initially mistook him for the King, her male counterpart. She was apparently in close communication with Maleldil, the supreme spiritual ruler of the universe, who helped the Green Lady understand that Ransom was from a different world and his ways were different.
Ransom and the Green Lady visited the Fixed Land, one of Perelandra's few true islands. Maleldil had made a rule forbidding the King or the Green Lady from dwelling there, or even spending the night. They saw an object fall into the sea. It was a spherical spaceship, of the same kind in which Ransom was kidnapped and taken to Mars. In fact, they soon meet his abductor, Weston, on the beach.
Weston's humanistic, scientistic beliefs had evolved into a mystical form of emergent evolution. He now believed in a cosmic Force that directed his actions. When the Christian Ransom attempted to refute this belief, he became angry and was possessed by the Force. His own personality was apparently consumed, and he became the Un-Man. He escaped to the floating islands and attempted to persuade the Green Lady to violate Maleldil's law. While Ransom was there to refute his arguments, the Un-Man was relentless and had no need for sleep, and Ransom realized he could not keep up his side of the argument indefinitely.