Cambridge University-educated Maurice Allingham owns an inn, The Green Man, which is haunted by a seventeenth century clergyman believed responsible for two horrible murders. The action takes place over four days, which Allingham recalls one year later.
He encounters a mysterious red-haired woman who disappears. No one else sees her. The same evening, Allingham’s father, who seems to have seen something, dies of a stroke. Before falling asleep that night, Allingham hallucinates about a tree in the shape of a man and with the ability to walk. Allingham had earlier asked Diana, his best friend’s wife, to meet him for a tryst the next day. Thus, the tone of the novel is set in this first chapter: the possible existence of ghosts, the foreboding of the Green Man, the intrusion of death, and the alcoholic Allingham’s libidinous obsession.
The protagonist becomes increasingly aware of the ghostly presence of Dr. Thomas Underhill, whom only he can see. A trip to the fictional St. Michael’s College at Cambridge produces Underhill’s diary, which reveals the wizard’s use of magical illusion to frighten very young village girls into submitting sexually. Later that night, Allingham and Diana dig up and open Underhill’s casket, where they find a sheaf of papers. The following morning Allingham reads them and discovers precise instructions for how to achieve a meeting with Underhill.
Kingsley Amis leads the reader through a series of partial climaxes. Most immediate is Allingham’s meeting with God as a well-dressed man of about twenty-eight. Despite Allingham having characterized himself as a hardened unbeliever, God directs him to eliminate Underhill, leaving the means up to Allingham but suggesting that he obtain help from the church. The next climax, the meeting with Underhill, involves the wizard producing a series of lewd and horrific illusions. Allingham, using a crucifix, breaks the spell in time to prevent the Green Man from destroying his daughter, which Underhill planned to do after realizing he could not ravish her. The final climax is seriocomic. Allingham persuades an unbelieving Anglican priest to exorcise Underhill from the inn.