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.
Ransom and the Green Lady were exploring the Fixed Land, Perelandra's only solid landmass, when they observed a spherical object fall into the sea. On the beach, Ransom was surprised to find his nemesis Weston, who had abducted him to Mars (in Out of the Silent Planet); likewise, Weston was surprised to see Ransom. Maleldil, the spiritual ruler of the Solar System, has forbidden the Green Lady and her male counterpart, the King, from spending the night on the Fixed Land. Therefore, Weston allowed her to leave, but he kept Ransom on the beach at gunpoint.
Weston told Ransom that his philosophy had changed since they were on Mars. There, he had been a materialist and a colonialist, believing that it was humanity's destiny to expand to the stars. Anything that stood in their way, including the rational inhabitants of Mars, was expendable. Since then, his beliefs had evolved to embrace emergent evolution directed by an impersonal, amoral spiritual essence he called the Force. In spite of Ransom's warning that all spirits are not good, Weston called this Force into himself, apparently allowing himself to be taken over by it, before lapsing into a coma. Left alone on the Fixed Land, Ransom slept on the beach.