On a far distant planet in a far distant galaxy, evolution has taken a different turn than on Earth. On Umane-Bas a species becomes human, able to think and dream, in reward for an act of great heroism or sacrifice by a member of the species, an act understood to be beyond the capabilities of that species. To the inhabitants of Umane-Bas, this transformation to human status is known as Radiance. A boy and his dog from Earh, fifteen year old Keeper and Jagger, his Irish setter, arrive in the midst of a war between the city states of Shaldalene and Ravenscrag, and discover that they have been brought to Umane-Bas to preside over the Radiance of the horses, known on Umane-Bas as Striders. Not everyone wants the Striders to become human, and they try mightily to prevent Keeper and Jagger from reaching the predicted time and place. They do, with great difficulty and after much adventure, reach the time and place, and the Radiance of the Striders occurs at the height of the final cavalry battle between Shaldalene and Ravenscrag. Keeper's journey of discovery teaches him that destiny comes in many guises, that one's destiny is invariably linked to one's choices, and that wisdom and goodness and courage are not solely the province of a particular species.