In 2010, Ciara Fisk cuts her finger on a page of her book.  A drop of blood spills from the cut and lands on a certain pebble, one of many that form the rocky bed of the stream in front of her.  The pebble glows with energy, and suddenly something is released: a strange man with horns, fangs, and black wings, calling himself Mordecai.  He is a demon, he says, and he is loyal to her.  Except that he thinks she is someone else entirely, a girl who died centuries ago, the girl who summoned him last--and who imprisoned him in the first place.

In the world of Familiar, demons are earth spirits who make contracts with witches.  These contracts create a symbiotic relationship between the witch and the demon: the former supplies the will, the endurance, the stamina needed for spellcasting, while the latter provides the power, the efficacy, the big bang, if you will.  Any witch may summon a demon, if he or she has will enough, and anyone may become a witch.  It's thought that their practices were once the norm, eons ago, when the soil was a little more fresh on the ground--but those days are so long gone that they might as well have never existed. 

Presently, witches operate in relative secrecy.  To the few others who know that real magic exists, witchcraft is an abomination, and the demons are vile scions of hell, sent to sow chaos and ruin in the world.  The Vox Dei is a militant organization dedicated to exterminating demons and either converting or killing the witches who summon them.  They are opposed to all supernatural forces in the world, and view themselves as holy regulators, blessed by God with the necessary but unenviable mission of keeping the world pure.

Ciara must deal with the resurgence of the Vox Dei, with the mysteries of her small, strange town, and, most of all, with the demon she's just awakened.  Why was he imprisoned by her ancestor?  Why are some of her best friends suddenly withdrawing from her?  And why is so much magic done with blood?