In a small English town after World War II, young academics and newlyweds Mark and Jane Studdock eat dinner separately, as the usually do given their strained relationship. Jane is at home, reflecting on her troubling dreams of a prisoner losing his head, while Mark is at Bracton College (part of the University of Edgestow), where he works as a sociologist.
Mark enjoys being part of his friend Curry’s exclusive clique at Bracton, but he finds himself charmed by the newcomer Lord Feverstone. Feverstone argues in a College Meeting in favor of selling the ancient and culturally significant forest Bragdon Wood to the National Institute of Co-ordinated Experiments (N.I.C.E.). The College Fellows agree to the sale, and Feverstone persuades Mark to tour the N.I.C.E. and consider a position there. At the N.I.C.E.’s headquarters in Belbury, Mark meets Deputy Director John Wither, a vague old man who evades the question of what Mark’s job at the N.I.C.E. would be. He also meets Miss Hardcastle, also called “the Fairy,” who runs the N.I.C.E.’s private police force, and William Hingest, another Fellow of Bracton who is planning to leave the N.I.C.E. The next day, Hingest is murdered.
Meanwhile, Jane tries to understand her increasingly strange and upsetting dreams. Her friend Mrs. Dimble brings her to a Manor at St. Anne’s-on-the-Hill, where a doctor named Miss Ironwood explains that Jane’s dreams are in fact prophetic visions.
Mark is pressured into accepting the job at the N.I.C.E. and tasked with writing propaganda for the organization. The N.I.C.E. seeks to manipulate the public, who are already uneasy because Miss Hardcastle’s police force has been instigating riots around Edgestow.
On a visit to Edgestow, Jane recognizes N.I.C.E. member Professor Frost from her visions, and she is so upset that she goes to St. Anne’s for help. Her new friends introduce her to their leader Mr. Fisher-King, who reminds Jane of mythological kings, and he advises her to embrace humility and obedience in her marriage. Jane returns home, but on the way she is caught by Miss Hardcastle, who tortures her. When Jane escapes, she goes back to the Manor to recover, and learns that Mr. Fisher-King’s real name is Elwin Ransom. He has returned to Earth following his interplanetary travels to serve the eldils (angelic beings) and save humanity.
The leaders of the N.I.C.E. plan to establish an emergency government at Edgestow, ostensibly in response to the riots they caused. Mark converses with other N.I.C.E. members Dr. Filostrato and Reverend Straik about the N.I.C.E.’s plans to elevate humanity beyond the natural world by separating it from organic life. Filostrato and Straik introduce Mark to the Head, a severed head that leads the N.I.C.E.
The residents of the Manor, who refer to themselves as Logres after the Arthurian kingdom, discover that the wizard Merlin is sleeping under Bragdon Wood. Jane leads a search for Merlin while the leaders of the N.I.C.E. discuss their own plans to find the wizard. They frame Mark for Hingest’s murder and use the threat of execution to keep Mark in their control, though this blackmail finally convinces Mark that he can’t trust the N.I.C.E. Professor Frost explains to Mark that the Head is possessed by a Macrobe, which is what the N.I.C.E. calls Earth’s evil eldils. Frost is in league with the Macrobes to instigate a war that will wipe out the undesirables of society.
Merlin wakes from his slumber and arrives at the Manor. At the same time, the N.I.C.E. capture a tramp, who they believe to be Merlin, and keep him imprisoned as he refuses to speak to them. Frost leaves the tramp and Mark locked in the same room, and the two become friendly. Meanwhile, Merlin pledges service to Ransom, in awe of Ransom’s connection to the eldils, and Ransom prepares to summon the Oyérsu (eldils that protect planets). Before he does, Jane has a vision of the Oyarsa of Venus, who imparts the message that Jane must accept her role as a Christian wife. Jane then experiences the presence of a divine Person that is so sublime it inspires her to serve God.
Ransom summons the Oyérsu of Mercury, Venus, Mars, Saturn, and Jupiter, and the Oyérsu channel their power into Merlin. Merlin goes to Belbury and inflicts the N.I.C.E. with the curse of Babel, driving them mad with the inability to understand one another’s speech. He also sets free the animals and prisoners the N.I.C.E. kept captive for experiments, which includes Mr. Bultitude, a bear who lives at the Manor. As some of the N.I.C.E. members kill each other, Wither, Filostrato, and Straik flee to the Head, who demands Wither sacrifice his companions. Wither obeys, only for Mr. Bultitude to burst in and kill him. Professor Frost lights the building on fire, killing himself. Feverstone escapes but dies in Edgestow when an earthquake destroys the town.
Merlin sends Mark to St. Anne’s. However, Mark is so ashamed of himself that he doubts Jane will want to be with him, so he avoids the Manor. Mrs. Dimble’s husband Mr. Dimble explains to the members of Logres the significance of their society: Logres represents the honorable tradition of England, while Britain represents England’s dark side. Ransom reveals that he will return to Venus to heal the wound he acquired fighting evil. Around the Manor, animals begin to mate, and Ransom notes that Venus is over St. Anne’s. He sends Jane to look for Mark, and though Mark believes himself unworthy of Jane, Jane looks forward to reuniting with him.