Thomas Covenant is a young author whose world is turned upside-down when he is diagnosed with leprosy. After six months' treatment and counselling in a leprosarium, he returns home to find himself alone, divorced by his wife Joan, and outcast from his community. His son Roger has been taken to live with his ex-wife. On a rare trip into town, he is accosted by a beggar. The beggar refuses Covenant's offers of charity, including his white gold wedding band, makes several cryptic pronouncements, and leaves Covenant with the admonition to "be true." Disturbed by the encounter, Covenant stumbles into the path of an oncoming police car and is rendered unconscious.
He wakes to find himself in "the Land", a classic fantasy world. He first meets the evil Cavewight Drool Rockworm, who has summoned him to the Land with the power of the enchanted Staff of Law. Drool has been instructed and manipulated by a malevolent incorporeal being who calls himself "Lord Foul the Despiser". Foul reproaches Drool for his arrogance and transports Covenant to Foul's demesne. Addressing Covenant as "groveler", Foul taunts him with a prophecy that he (Foul) will destroy the Land within 49 years; however, if Drool isn't stopped, this doom will come to pass much sooner. He tells Covenant to deliver this message to the rulers of the Land, the Council of Lords at Revelstone, so that they can make preparations to combat Drool Rockworm and recover the Staff of Law.
Covenant is transported again and wakes on Kevin's Watch, a tall finger of rock attached to a mountain overlooking the Land's southernmost region. He meets a girl named Lena, who uses a special mud called hurtloam to heal the injuries from his fall. Covenant is shocked to discover that the hurtloam has also cured his leprosy. This is only the first example Covenant will see of the Earthpower: a rich source of healing energy present throughout the Land. Covenant's loss of two fingers on his right hand, a consequence of the failure to promptly diagnose his leprosy, causes him to be identified by Lena as the reincarnation of Berek Halfhand, an ancient Lord who saved the Land from Lord Foul during a war which occurred in the Land's distant past. His special identity is seemingly confirmed when Lena's mother Atiaran identifies Covenant's white gold ring– in his world a plain wedding band, which he had been emotionally unable to discard notwithstanding his divorce – as a token of great power in the Land.
Believing that he is unconscious from his collision with the police car, and therefore experiencing a fantastical dream or delusion, Covenant refuses to accept the reality of the Land. Appalled and indignant at the expectations the people of the Land have for him as their new-found saviour, he gives himself the title of "Unbeliever."
He is also unprepared for the sudden restoration of his health, which cures the impotence brought on by his leprosy. This, and his mental turmoil over the reality he feels but does not believe, drives him into a frenzy, causing him to rape Lena, an act which will be pivotal to all that follows. When Lena's friends and family learn of what happened to her, they are barely able to comprehend the enormity of or reasons behind this crime, but the Oath of Peace to which they are sworn forbids them from taking vengeance.
Atiaran, with great chagrin, guides Covenant to the Hills of Andelain, a region of the land where the Earthpower is especially strong. There she entrusts Covenant to the care of Saltheart Foamfollower, one of the Unhomed Giants, who are allies of the people of the Land. The Giants, a seafaring people who live on the eastern coast of the Land, have a strong understanding of the Earthpower, especially as it relates to the Sea and other waters. Foamfollower is able to sail his stone boat up one of the great rivers of the Land to Revelstone, the Lords' mountain fortress.
Covenant delivers the message of Lord Foul to the Lords. He is invited into their council as an "ur-Lord" because of his connection to Berek, and his white gold ring, which the Lords recognize as having the power to unleash the "wild magic" which may be the key to defeating Lord Foul. Despite the obvious danger, the Lords decide to make an effort to wrest the powerful Staff of Law from Drool's evil grasp. Rather than waging an all-out war, the Council sends four Lords and a band of forty warriors to attempt to infiltrate Drool's lair at Mount Thunder.
Led by High Lord Prothall, and accompanied by the Lords' sleepless and ageless protectors the Bloodguard, and the Giant Foamfollower, the Lords' party sets out eastward. Covenant joins them in the hope that the recovery of the Staff of Law will somehow assist in his return to his "real" world. Along the way, Covenant attempts to come to terms with whether or not to believe in the reality of the Land. He also attempts to redeem himself for his outrage against Lena by commanding one of the Ranyhyn, the wild, free and intelligent horses of the eastern Plains of Ra, to do homage to her yearly. The Ramen, a tribe of humans who dedicate their lives to care and protection of the Ranyhyn, though repulsed to see their equine companions under Covenant's compulsion, agree to assist the quest on the last leg of its journey.
In the end, at the cost of the deaths of many of their companions, the Lords succeed in penetrating Mount Thunder and seizing the Staff, temporarily securing peace for the Land. Covenant destroys Drool Rockworm and saves the surviving members of the party by using the wild magic of his ring to summon the Fire-Lions, creatures of living lava which issue from the peak of Mount Thunder, although he does not fully control or even understand his power.
After the death of Drool, who had used the Staff of Law to summon Covenant to the Land, Covenant feels his physical body fading away, loses consciousness, and wakes up in his own world, a leper once more.