Powerful Circe, daughter of the sun-god Helios, is sad to see Odysseus, King of Ithaca, depart from her island, Aeaea-but her heartbreak is eased after dolphins take her to Delos, where she explores a new love relationship.

Circe has a strained relationship with her mother, Perse, but when she finally listens to Perse's encouragement to seek out the amphibian god Glaucus, she's glad she's heeded her advice. Together, the two embark on underwater adventures, and Circe shares with Glaucus her knowledge about the healing and harmful power of herbs. While in Delos, she also meets and befriends Skylla, a local beauty with whom Glaucus is enthralled, although the girl is indifferent.

Circe eventually returns to Aeaea, but one day she learns, upon consulting her scrying mirror, that there is trouble in Delos that requires her immediate action. In the turbulent world of gods mingling with mortals, our heroine shifts shapes, flies, and uses her superpowers to reverse the course of evil.

In a tangle of love, hate, vengeance, and the final righting of wrongs, a cast of irresistible characters weaves an adventure laced with beauty and terror in An Unexpected Ally-a newly woven set of tales that brings to life ancient Greek myths and revives issues familiar to contemporary readers.