The book is one of 5 prequels to Bradley's popular fantasy novel Mists of Avalon. The characters of Ancestors of Avalon have appeared earlier in Bradley's fantasy novel The Fall of Atlantis. Ancestors is the missing link to the implied connection to the mythical Atlantis in Bradley's Avalon-series.
2500 years BC, Tiriki, Priestess of Light and of the Earth-goddess Ni-Terat, and her husband Micail, Priest of Light and Prince of the Atlantean state Atharrath, have to save themselves from the destruction of Atlantis and are forced to board different ships to the Isles of Tin (Britain). When they finally reach the British coast, they are far away from each other and they both believe that the other one is dead.
Tiriki and other Atlanteans who came with her, settle down in the swamplands surrounding the Holy Mountain (which is later going to be called the Isle of Avalon). She realizes that the cult of the Great Goddess is much stronger here than it was in Atlantis, so she and her companions start living with the indigenous people and build up a new religion, where the Atlantean knowledge and the Old Faith of the British people merge.
Micail and Tjalan, Prince of Alkonath, on the other hand, try to rebuild the lost glory of Atlantis and start building a huge stone circle -which will later be known as Stonehenge - in order to turn the people of Britain to slaves by using its tremendous powers.
When Tiriki and her followers finally come in contact with the other Atlantean settlement, conflicts arise immediately.
Tiriki and Micail are early incarnations of the characters Igraine and Uther Pendragon from Mists of Avalon, and of Gawen and Sianna in Lady of Avalon.