Spoiler alert: This post is part of an in-depth discussion of Perelandra by C. S. Lewis, which will inevitably reveal key plot points. You may well have a lot of reading ahead of you.
Ransom, brought by the Oyarsa of Malacandra to the planet Perelandra, encountered his enemy, the physicist Weston, there. Only Weston was no longer himself, having become possessed by the "Force" he reveres—really Satan or one of his fallen angels—and become what Ransom termed the "Un-Man." Weston was there to tempt and corrupt the Green Lady, the unfallen Eve of Perelandra. He did this by relentlessly trying to appeal to her sense of vanity and to persuade her that the one law on Perelandra, against dwelling on its fixed island, was made to be broken. Ransom attempted to dissuade her from heeding Weston. Fortunately, the Lady seemed resistant to the Un-Man's persuasive skills.
After days of this, Ransom sits against a tree and questions divine justice. He has made no headway against the Un-Man. If the Enemy was there in person, where is the representative of Maleldil to counter him? The answer that comes back to him in the dark is that, of course, he is Maleldil's man: an idea that he almost dismisses as blasphemy. But he realizes that Maleldil has been with him all along. His own arrival on Perelandra was as miraculous as Weston's. The fate of Perelandra is left in God's hands—and he and the Green Lady are those hands.