After seeing his friend, Dr. David Throckmartin, borne away in a pillar of light radiating from the full moon, Dr. Walter T. Goodwin, acting on information conveyed to him by Throckmartin before his disappearance, hastily forms a small expedition to the island of Nan-Matal in the Carolines. This is the site of ancient ruins and the previous disappearance of Throckmartin’s wife and companions.
In the company of Larry O’Keefe, an Irish adventurer, and Olaf Huldricksson, a Norwegian sea captain whose wife and child had also been taken by the Dweller in the Moon Pool, Goodwin enters the Dweller’s cavern. Members of the party overcome a treacherous German scientist, Von Hetzdorp, who ambushes them, and then find themselves projected downward through an apparently endless tunnel to an underground world. This world, Muria, is inhabited by descendants of the lost continent of Lemuria and is ruled at a distance by the three Silent Ones, who fear the growing, maleficent power of the Dweller, whom they created. The Silent Ones are prevented by their love for the Dweller from opposing him directly.
In the city of the Dweller, O’Keefe is infatuated with the dark-haired Yolara, priestess of the Dweller’s cult, but haunted by dreams of a goddess with golden-brown hair. Yolara’s chief supporters are a race of green dwarfs who plan to enslave the peoples of the upper world with the power of the Dweller and weapons that surpass anything known in the upper world. Accompanied by Rador, a sympathetic ally, Goodwin’s small party leaves behind Von Hetzdorp, who has cast his lot with the people of the Dweller. They escape through a portal that admits them to the city of the Silent Ones. There they find Lakta, the goddess of O’Keefe’s dreams.
In an epic battle between the Yolara’s forces and the giant batrachians who inhabit the city of the Silent Ones, the Dweller is destroyed and his forces are routed. As Goodwin leaves for the upper world, followed by Larry and Lakta, he is surprised by a crazed Von Hetzdorp, who pulls Goodwin with him into a stream where Goodwin loses consciousness. He awakens to find himself in the island ruins, with Von Hetzdorp dead and the moon door closed forever to the two lovers.