The story begins in 296, when the general Constantius comes to Britain and falls in love with the British princess Eilan, called by the Romans Helena, daughter of the High Priestess in Avalon. The High Priestess sees in this relationship a chance to link the ancient royal line of Britain with that of the emperors. Helena, who both loves her mother and resents her power, is married to the young Caesar and the priestesses of Avalon aid the Romans with their magic.

Constantius' success brings him the Emperor's favor, and he is made a Caesar, but his rise spells the end of their marriage, for the emperor forces him to set Helena aside and marry a patrician girl. Grieving, Helena returns to Britannia, and Avalon.

As Constantine struggles for political supremacy, Helena tries to understand the changes that are taking place in the Empire. When he makes Christianity the state religion, she is forced to appear to comply. She goes on pilgrimage to the Holy Land, still seeking understanding, and realizes at last that there is a truth behind the forms of religion that transcends both the old religion and the new.