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After spending three years among the yogis, Frank Chandler, having acquired hypnotic powers of the will, is given the name Chandu and told that Fate has appointed him to conquer the evil madman Roxor.  With a death-ray invented by Chandu's brother-in-law, Robert Regent, whom he has captured, Roxor plans to destroy civilization so that he will be proclaimed the master of the world.

After Chandu rescues the Egyptian Princess Nadji, whom he had loved in Paris three years earlier, Regent's daughter Betty Lou is abducted and taken to a slave market to force her father to divulge the secret of the death-ray to Roxor.  By creating a double of himself, Chandu rescues Betty Lou, but he and Nadji are captured at an oasis when Roxor's cohort Abdullah, who hates but also greatly desires Nadji, throws tear gas in Chandu's eyes.  Imprisoned, Nadji and Chandu confess their love for each other before Roxor has Chandu put into a mummy's coffin and sunk underwater.

Meanwhile, when Roxor has Regent watch as the floor underneath his family gradually collapses, Regent agrees to prepare the death-ray for Roxor to save his family.  While Roxor imagines the destruction of the great cities, Chandu escapes and paralyzes him with his gaze.  Chandu rescues Nadji from Abdullah.  The death-ray, which Regent had fixed to explode to kill himself and his family, destroys Roxor's temple.  After Regent, his family, Chandu and Nadji escape, Chandu makes the clouds cover the moon and kisses Nadji.