A visionary astronomer, Dr. Janos Rukh, has invented a telescope that can look far out into deep space, into the Andromeda Galaxy, and photograph light rays that will show the Earth's past. He has theorized about this being possible for some years, much to his discredit among his fellow scientist-colleagues. Looking at the remote past on a planetarium-like dome in his lab, two of those ardently skeptical scientists, Dr. Benet and Sir Francis Stevens, watch a large meteorite smash into the Earth a billion years ago, in what is now the continent of Africa. Amazed by Rukh's demonstration, the pair invite him to go on an expedition to locate the impact site.

Rukh finds the meteorite, but is exposed to its unknown radiation, now dubbed "Radium X". This causes him to glow in the dark and to make his mere touch instantaneous deadly to any living thing. The exposure also begins to warp his mind. Returning to the base camp, he entreats Dr. Benet to devise a means of neutralizing Radium X's poisoning effect. Benet develops a serum that holds the lethal element's toxicity at bay, but Rukh must take regular doses of the antidote or he will revert to being a luminous killing machine. Rukh returns to his jungle base and learns from Benet that this situation has been complicated by the romantic relationship between Rukh's wife, Diana, and Ronald Drake, the nephew of Lady Arabella Stevens, Dr. Stevens' wife.

Benet takes a piece of the meteorite back to Europe, where he modifies its effects to help people, including curing the blind. Working along similar lines, Rukh cures his mother's blindness, but in spite of her warning, he goes to Paris to confront Benet and the others. There, he pretends to acknowledge his wife's new relationship with Drake, but in reality, it is the first step in his plan for revenge. Rukh murders a Frenchman he closely resembles, making it appear that he has died and been rendered unrecognizable due to an accident with Radium X.

Believing the deception, Diana marries Ronald. Rukh now begins to use his radiation poisoning to kill off the expedition members. He marks each death by disintegrating a single statue on the exterior of a church across from where he is hiding. Each time, he focuses the radiation through a window using a raygun-like device. He manages to kill both Stevenses before the police realize what is happening. Dr. Benet helps them set a trap by convening a scientific conference at his home to discuss Radium X, but Rukh secretly gains access and kills Benet. He has saved his revenge on Ronald and Diana for last but finds himself unable to kill his former wife. This hesitation brings him to a confrontation with his mother, the most important woman in his life. She has foreseen her son's growing madness and smashes the last of his antidote bottles in order to stop him. As the Radium X begins to consume him from within, Rukh jumps from a window. He disappears in an explosive flame, having been vaporized before reaching the ground.