A beautiful young European girl, Carol, Olga Schoberová is drawn through mental telepathy to the ancient lost city of Kuma, there to become the reincarnation of its lost former ruler, Ayesha, and consort of her predecessor's lover, Kallikrates John Richardson. In return, Men-Hari Derek Godfrey, a member of the Magi, the ancient Chaldean race of wise men, will also be allowed to enter the sacred flame and become immortal, which will expand his already formidable mental powers to the point where he will be able to take over the entire world. To achieve this, however, he must bring Carol to Kuma before the sacred flame is ignited during a once-in-a-lifetime astronomical alignment. Men-Hari therefore uses his powers ruthlessly to compel Carol to come to Kuma.
Men-Hari is opposed by his father, Za-Tor Noel Willman, longtime leader of the Magi, and by Dr. Phillip Smith Edward Judd, a psychiatrist who meets and falls in love with Carol during her journey. In the course of their travels, Carol and Phillip are separated. Kassim André Morell, a local mystic, attempts to break Men-Hari's control over Carol, but Men-Hari learns of his efforts. Partly at Kallikrates' bidding, Men-Hari wrests the leadership of the Magi away from Za-Tor, and leads the rest of the Magi in a forbidden occult ritual to overpower and destroy Kassim. Shortly thereafter, Carol and Phillip are reunited, and they continue their journey to Kuma.
Upon their arrival, Carol is welcomed, but Phillip, whom Men-Hari rightly perceives as a threat to his evil scheme, is imprisoned. Za-Tor comes to Phillip and discusses the situation with him, and comes to realize the danger in Men-Hari's plot. He agrees to do whatever he can to help Phillip and then departs. Sharna Danièle Noël, one of Kallikrates' servant girls, and who is in love with Kallikrates, helps Phillip to escape, while Za-Tor speaks to his assistant in an effort to incite a rebellion against Men-Hari. The plot succeeds to some extent, and Phillip arrives at Kallikrates' chambers just as the sacred flame is ignited. Before Carol can walk through the flame, however, Phillip desperately calls out to Carol, even as he is seized by Kallikrates' guards. Phillip's cries succeed in breaking Men-Hari's control over Carol, and at that moment Za-Tor confronts Men-Hari, explaining his plot to Kallikrates. Realizing that Za-Tor is telling the truth, Kallikrates orders that Carol and Phillip both be released. He also bars Men-Hari from entering the sacred flame, and denounces him as the power-mad traitor that he is.
Stung to fury by the frustration of his plot, Men-Hari stabs his father in the stomach with a long dagger. As Phillip rushes to Za-Tor's aid, Men-Hari attempts to kill him, but Kallikrates' guards kill Men-Hari with their swords at their king's command. Kallikrates, now despondent beyond all reasoning, then commits suicide by re-entering the sacred flame, despite Sharna's efforts to stop him. As the others watch in horror, Kallikrates ages hundreds of years in a matter of seconds, then dies and crumbles to dust. As Phillip and Carol leave the chamber and head for the main entrance to the city, Za-Tor revives just long enough to pray for Kuma's destruction, since its people have now become evil beyond all hope of rehabilitation. In direct response, the sacred flame explodes outward, and an earthquake begins to tear the city apart. Phillip and Carol just barely make it out of the entrance before the city crumbles and collapses, killing everyone within. As the two lovers begin making their way back to civilization, the last fragments of the giant sculpture of Ayesha that stood above the entrance are engulfed by the sacred flame, signifying the end of Kuma and the Magi for all time.
