There are three races of immortals in the universe; the Lumen who represent the good in it, the Fuscus who represent the darker forces and the Trutina who maintain the balance. To the mortals they are known simply as the Guardians.  On one world, Mundus, the immortals have vanished leaving the world open for a new race, the Probus, to gain a foothold. Elora, one of the Trutina, is sent to restore the balance and find out what happened to the other immortals so that once again they can keep watch over their charges to maintain that balance.