The story is told through the eyes of Nathaniel Wingate Peaslee, an American living in the first decade of the 20th century, who is "possessed" by a Yithian. He fears he is losing his mind when he unaccountably sees strange vistas of other worlds and of the Yithian library city. He also feels himself being led about by these creatures and experiences how they live. When he is returned to his own body, he finds that those around him have judged him insane due to the actions of the Yithian that possessed his body. While he was experiencing a Yithian existence in Earth's ancient past, the Yithian occupying his body was experiencing a human one in the present day.