Before he dies of radiation poisoning, scientist John Vincey summons his nephew Leo to his home and confides in him the nature of his dangerous experiments. Supported by his partner, Horace Holly, John reveals that for years he has been trying to discover a naturally occurring but extremely rare radioactive element he believes has the power to preserve life. His belief is derived from a five-hundred-year-old letter, handed down through the Vincey family, which states that while English ancestor John Vincey and his wife were living in a Siberian settlement, they came upon a remote but civilized area, a glacial region of volcanic character. There, they learned about the "flame of life," the radioactive element, and the legend that states "he who stands therein comes forth unharmed to triumph over death." According to the letter, which was written by the wife, Vincey found the flame, but was murdered because he refused to renounce his marriage vows. The wife escaped from the north and made her way to Poland, where, upon her death, she bequeathed her son the letter and a gold statuette taken from the tribe. John, in turn, gives the statuette to Leo, who bears a close resemblance to his ancestor, and begs him to travel with Holly and locate the "flame of life." After Leo accepts the mission, John quietly dies. Sometime later, Leo and Holly reach a remote Arctic outpost and are taken in by the brutish Dugmore and his daughter Tanya. Impressed by the gold in the statuette, Dugmore forces Leo and Holly to take him on the last leg of their trip by threatening to turn any potential native guide they may hire against them. Over Leo's objections, Dugmore also insists that Tanya accompany them, and soon Leo and Tanya fall in love. When the expedition reaches the glacial Sugel Barrier, just north of where the legendary civilization is supposed to be located, Dugmore stumbles on an ice-encrusted corpse laden with gold. Although Holly warns him against using his ax to retrieve the gold, Dugmore greedily begins chopping away at the ice and causes an avalanche. Dugmore is killed by a wall of snowy ice, while Leo, Tanya and Holly are cut off from the expedition. The avalanche does, however, expose a passage through the barrier, which leads the trio into a volcanic cave. There, they are surprised by a tribe of cave dwellers, who try to make a human sacrifice out of Holly. When Leo shoots his rifle at the cavemen, a fight ensues and Leo is knocked unconscious. Before the cavemen can capture their visitors, another group, led by Prime Minister Billali, emerges from the shadows and claims the trio for itself. The trio then is taken into the bowels of the cave to a land called Kor, where Leo is laid at the feet of She, Hash-A-Mo-Tep, the mysterious, all-powerful monarch. Upon viewing the still unconscious Leo, the beautiful but merciless She becomes agitated and refers to him as John Vincey. Later, She tells Leo that she is immortal and that he is the reincarnation of Vincey, the man she had loved passionately but had killed in a fit of jealousy, the man whose return she has been expecting for 500 years. After She assures him that the secret of immortality will be his, Leo tells Tanya and Holly to leave without him. Although Tanya tries convince Leo that his love for She is unnatural, she is unable to sway him. Out of jealousy, She then orders Tanya to be used as a human sacrifice in an upcoming ritual. Leo remains under She's romantic spell until he realizes that the veiled sacrificial victim is Tanya. Leo rescues Tanya as she is about to be dropped into a holy well and flees with her and Holly. Jumping across a dangerous precipice, they leave their pursuers behind and stumble onto She's sacred temple. After She admits that they have found the source of her immortality, she insists that Leo immerse himself in the radioactive flame. When he refuses, She enters the flame twice, but instead of rejuvenating her, it causes her to age rapidly until she collapses and dies. Back in England, Holly offers scientific explanations as to why She was killed by the flame, while Tanya is sure that they were saved by true and natural love.