Story tells of the Great Race of Yith, an extraterrestrial species with the ability to travel through space and time. The Yithians accomplish this by switching bodies with hosts from the intended spatial or temporal destination. The story implies that the effect when seen from the outside is similar to spiritual possession.
The Yithians' original purpose was to study the history of various times and places, and they have amassed a library city that is filled with the past and future history of multiple races, including humans. Ultimately the Yithians use their ability to escape the destruction of their planet in another galaxy by switching bodies with a race of cone-shaped plant beings who lived 250 million years ago on Earth.
