Spoiler warning! Reading this summary may spoil the fun of reading the book.
The story opens in the voice of the narrator making a report to Satan on the defeat recently suffered by the forces of the dark in their attempt to destroy the ancient Fellowship of Magic. The narrator is Ambrose Bierce, the writer, now a special investigator assigned to the legions of hell. Bierce insists on making his report in the form of a fiction, since his investigation was hampered by the absence or unavailability of many of the principals involved.
Randall and Vanessa Campbell, powerful members of The Fellowship Of Magic, are pursued by the witch, Urthane LaVoisin, through the stormy streets of Bern, Switzerland, seeking sanctuary at the home of Randall's brother, Josiah Campbell. Vanessa is heavy with child and weak-willed. When she can no longer resist, she abandons her husband, her body and the baby, fleeing forever into another realm. Randall, exhausted, and burdened with his wife's body, attempts to warn his brother telepathically, but is defeated within sight of the refuge he sought. Josiah rushes into the storm to attempt the rescue of his family, but is too late. He carries off Vanessa's pregnant body, intending to try to save the child. He senses the presence of the witch. Once inside his residence, he sets wards against Urthane's entry, succeeding just in the nick of time. However, at that moment, the body goes into labor. Josiah prepares himself to transfer his mind to the body of his sister-in-law and give birth, but is startled to discover that someone else already has. The child gives birth to itself.
The boy child is newborn, but already strong-willed, telepathic and healthy. He is named Lee Randall by Josiah, whom he calls "the giant." He does not cry. Josiah calls an attorney in the United States named Thaddeus Soames, and asks him to find a place for the child where it can be kept safe. Arrangements made, Josiah uses psychic engineering to prepare the child for its future.
The witch, LaVoisin, celebrates her victory over Randall and Vanessa. She plots the next step in her continuing attack on The Fellowship of Magic.
Justice Ralph McGuire finds the infant on the steps of the County Courthouse in Fairfax, Virginia. The child is examined by Dr. Phil Evers and pronounced in fine health. The only clue to its identity is a note pinned to a blanket saying, "Lee Randall, I leave him in your care."
The baby is placed in the Fairfax Home under the care of Emma Sykes. Lee Randall is placed under the care of Emma's assistant, Jayne Richards, who grows fond of him. However, one afternoon, the child wounds himself on the pin of Jayne's nametag and she observes his eyes as they change from blue to green to brown to black just before passing out. She is terrified and calls for Phil Evers. He then observes the same phenomenon.
Josiah Campbell dreams of Urthane, a nightmare from which he awakens in panic and sweating with fear. We meet his servant, Franz, a mysterious figure who has mastered the art of invisibility and is Josiah's closest advisor and security chief. Franz briefs Josiah on Urthane.
The years pass and Lee Randall is found to have an eidetic memory and other strange abilities. He seems, for example, to possess psychokinetic powers. Finally, when Lee Randall is ten years old, a magic show is presented at the home by a magician called Kamazan the Great. After the show, Lee meets with the magician and is presented with a magic ring, told it is a gift which he must keep secret. Lee Randall discovers magic – not sleight of hand, but real ceremonial magic. He resolves to study it and become a practitioner. Meanwhile, he uses his abilities to avoid adoption.
Josiah is awakened from a sound sleep by Urthane LaVoisin, standing in his bedroom, threatening to take his life. He is bathed in a foggy gas, but the form of the witch turns out to be only a projection, a technological trick. Franz urges him to go immediately to his physician to be sure he has come to no harm.
Lee is anxious to get to the Public Library to begin his research, but the witch visits the home and tries to arrange to adopt him. The boy is able to elude her, and Emma is suspicious. When she departs, the witch throws away the necessary adoption forms, indicating the danger is over for the present.
Lee Randall finally gets to the library where he begins his studies. On the library grounds, he meets the man who will be his first teacher, a black herbalist and witch doctor named Missaboula Jabamia (Abu).
Bierce reports to Satan on the history of The Fellowship of Magic. We then follow Josiah as he travels magically to Egypt to preside over the funeral of a great mage named Keefer Bant. When he arrives in the buried pyramid in the ancient city of Kemet, he is horrified to witness the assembled wisemen collapsing and dying from an unknown poison gas. He is rescued at the last moment by Franz.
At the age of seventeen, Lee Randall falls in love with a girl named Karlyn. We learn that someone has broken into Abu's shop and into the home. Detectives are investigating and Lee is the most obvious link. We learn that Emma is a member of the Fellowship. On his way to Karlyn's home to pick her up for a date, Lee is followed by a shadowy figure.
At the Repository in Bern, Josiah is recovering from the attack at Kemet. He is watched over by the servant, Franz. He awakens to learn that his experience was not a bad dream, but reality. When he asks Franz about his chances, the demonlord is unable to be encouraging. Josiah has a vision and begins to formulate a plan.
Lee is hired to work at the library. After visiting Abu, he showers and dresses prior to his date with Karlyn – a date on which he asks her to marry him. She accepts. As he escorts her to the subway, they are again followed by the mysterious figure and, just as the train pulls in, the girl is pushed in front of the train. She is killed. Lee is taken home by Abu, in shock and devastated.
The witch celebrates her triumph.
Lee buries himself in work, spending long hours at the library. He is approached by the lawyer, Soames, who brings him a letter from Josiah. Lee learns his history. Untrusting, he examines Soames telepathically, but learns the man is telling the truth. Convinced, he quits his job and follows the lawyer to the bank, where he learns he is suddenly a millionaire.
Josiah has prepared his plans for the defeat of LaVoisin. He discusses the situation with Franz. Suddenly Josiah falls silent, and when Franz examines him, we learn that Josiah is dead – killed by the sting of a poisonous fly.
Soames tells Lee of his uncle's death and gives him another letter and other papers. Lee learns that he is to become the new Librarian of The Repository, and that the library will be moved from Switzerland to Virginia. Both Abu and Soames are as shocked as the boy to learn of Josiah's death. The building is selected, the renovations completed. Soames explains that the servant Franz and his crew will build the actual Repository in the basement, asserting that he will stay away out of personal fear of the demonlord.
Lee meets Franz and admits him to the building. The library is built, Franz says farewell. Lee calls Soames to tell him the task is done.
We finally see The Repository, a magician's library extrordinaire. Each area is dedicated to the study of a different field of real magic, from druidic magic through Voodoo. We meet Alibeck the magician, who is to be Lee's new tutor.
The witch, Lavoisin, continues her preparations. Needing a virgin sacrifice, she goes to the park where children are playing. While her servant hides in ambush, she approaches the male parent of a set of triplets, and seduces him into going into the woods with her. She commits a heinous act, then steals the oldest of the girls.
Lee Randall practices the craft of magic. We learn more about Alibeck and the Fellowship. Lee finally takes a night off and meets a beautiful women in a bar. She suggests they go for coffee. By the end of the evening, he invites her to his home above the Repository. Lee and the woman arrive at his home, and he senses that something is amiss. He has been drugged. The woman plies him with questions, sheds her reed dress, then seduces him. Lee awakens alone. He is muzzy and confused because there is no trace of his female visitor from the night before. There are no dirty dishes, there is no trash. Nothing indicates he ever saw the woman the night before. When he goes downstairs to the Repository to see Alibeck, he finds the man horribly murdered and mutilated. Lee calls the police, who arrest him. Soames is summoned and gains his release.
Soames, Abu, Judaheim and Lee plan a way to capture the witch in the act on video tape. Judaheim is the bait. Two detectives, ordered to cooperate by Judge McGuire, are present that night when everyone falls into a strange, undisturbed sleep. In the morning, Lee awakens first and discovers that Judaheim has been crucified on the upside-down cross in the Satanic chapel. LaVoisin is captured on videotape. Thus Lee is cleared.
Lee and Soames call up the demon, GAAP, and he tells them where to find the witch – in Winchester, Virginia. Soames drives Lee to the place and drops him off. Soames leaves and Lee confronts LaVoisin, who drugs him yet again and imprisons him.
Lee awakens in a place underground. He is introduced by LaVoisin to her servant, Torg, a huge, ugly, monstrous beast who salivates acid. Urthane tells him about her history, falling into the trap of reminiscences, then realizing that the boy has distracted her. Urthane admits to killing Karlin. She taunts him.
Torg guards Lee. There is something strangely familiar about the monster. Urthane comes to him and attempts to seduce him again, but he resists. She then has Torg bring in the young girl she stole. Lee offers to trade his life for hers and Urthane is amused. Lee realizes he cannot prevent the sacrifice. He comforts the child as best he can, then hardens his heart.
Lee is taken by Torg to Urthane in the middle of a pentagram. Here, she tells him, he is to be sacrificed to Satan. She conjures the dark lord, prepares to renew her contract with him.
Lee has finally been triggered, his latent abilities energized by the ring of power and Josiah acting from beyond the grave. He disrupts the ceremony at the critical point and LaVoisin is struck down while her Lord is humiliated and banished. Lee struggles with Torg and kills the beast. Then he sits beside the stricken Urthane, offering her the hope of forgiveness and resurrection. She declines three times and then crumbles into dust.
Lee returns to his home and awakens to discover a beautiful woman, who looks exactly like Urthane, only light instead of dark. She is Nimue, daughter of Merlin and she feeds him, prepares him to present himself to a gathering in the Repository below, and then Lee reports to the library. He is greeted by all the remaining mages and Merlin himself. Lee is swept away by the emotional fulfillment of realizing he has found a home at last.
