The Shadow Out of Time indirectly 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.