'A great scientist taps the strange powers of the mind, opening up a whole new science of Psycho-Physics. His mental powers augmented to superhuman capacity by a series of brain-potential-boosting cells, he constructs in the red-hot magma beneath the Pacific Ocean a great cube of force to act as a shelter for selected specimens of humanity against the doom threatening our race. He sends into the outer world his daughter, to bring back a man fit to rule beside her as King of Helioxenon.
'The amazed newcomer witnesses the marvels of the Hallucinators, the Great Machine, the Little Machine, the Memories, and all the diverse superscientific marvels of this world within the Earth. Not the least of Helioxenon's wonders is the Psychophone, with which the scientist keeps in touch with three different worlds of distant solar systems. Truly alien civilizations are described, and the conflict of psychologies involves the shattering of the lovers' hopes unless the icily intellectual beings of Triangulum can be made to see the beauty of human love. The story is a tense drama of interstellar intrigue, bustling with vast and weirdly original comcepts. In at least one respect it is superior to Smith's stories, for while they frequently descend to blood-and-thunder action, the thrills of "World D" are conflicts of minds and personalities. This is a thoroughly adult story.'
Source: Online Review