The novel is set in a parallel world in which the existence of psychic powers has permitted the development of witchcraft into a science; in contrast, the physical sciences have languished, resulting in a modern culture reminiscent of our eighteenth century. Witchcraft is hereditary but the ability to use it can be held by only one member of a family line at a time, being passed from mother to daughter at the daughter's loss of virginity. The daughter's lover then gains possession of her magical talisman, a jewel known as a "blue star", which enables him to read the mind of anyone he looks in the eye. The catch is that he retains access to this power only so long as he keeps faith with his witch lover.
The empire in which the action is set is comparable to the Austrian one in our own history. The government bans witchcraft, which merely serves to drive its practitioners underground, where they can fall prey to the use and abuse of unscrupulous powerful or ambitious individuals. The protagonists are Lalette Asterhax, a hereditary witch, and Rodvard Bergelin, an ordinary government clerk who has been recruited into the radical conspiracy of the Sons of the New Day. Rodvard, though attracted to the daughter of a baron, is commanded by his superiors to seduce Lalette instead to gain the use of her blue star in the furtherance of their revolutionary aims. The witch is no more truly enamored of him than he is of her, but both fall in with the scheme for their own reasons, unaware of how much they are simply pawns in the larger scheme of things.
Everything soon goes bad, and the couple is forced to flee the empire. Various adventures and complications ensue as they stray into one cause or acquaintance after another, gradually growing beyond their shallow, selfish roots into a greater understanding.