Meet Bo. She'sjust your average, gorgeous, butt-kicking, life-force-sucking bartender on the run. Her inhuman hunger for human energy has plagued her for the past ten years and is a secret she keeps close, but when she uses her power to stop a rape, her dark identity is exposed. The girl, a pickpocket named Kenzi, won't let Bo slip away without an explanation for the glowing blue light that escaped the man's body when she kissed him. Left with no other choice, Bo takes Kenzi under her wing in order to protect Kenzi and her own secret.
But Kenzi wasn't the only one to learn of Bo's...ability. Bo left the crime scene in a hurry, and the rapist's body, disfigured from his encounter with the paranormal, was discovered by a local detective named Dyson, who is also more than he seems. Just as Bo and Kenzi are about to jump town, Dyson rolls up in a gray van with his partner and smuggles Bo away to an unknown location. Kenzi frantically tries to track Bo, but she may be too late.
In a secret headquarters across town, a mysterious man called The Ash demands to know if she's allied with the Light or the Dark, and how she remained hidden for so long, but none of this means anything to Bo. The Ash doesn't believe her, nor does a woman called The Morrigan, who storms into the party unannounced with the threat of death on her lips.
After hours of interrogation, Bo is released for examination by a sympathetic doctor named Lauren, who tells her that she, Bo, is a member of a large group of paranormal beings called Fae. Lauren goes on to say that Bo will have to choose to belong to the Light or Dark Fae, and that they can teach her how to control her power. Bo distrusts this hierarchy, however, and uses her powers of seduction - another strange trait of the succubus - to lure Lauren away from the facility.
But the escape is foiled by The Ash and The Morrigan, leaders of the Light and Dark Fae, respectively, who demand that Bo undergo a test and declare her allegiance to one of their clans. Dyson, who believes Bo when she says that she doesn't know anything about Fae or succubi, tries to stop them, knowing that The Test - in which Bo will have to battle two Under Fae to the death - will most likely kill Bo. The clan heads are unmoved, however, and Bo must be tested. All Dyson can do is insist that Bo use her powers to take some of his strength. Because he is a different kind of Fae, she is able to use him without ending him, an experience that fills Bo with a thrill unlike any she's ever known.
Bo's first opponent is strong but slow, and Bo is able to fell him quickly. But her next opponent is a Fae of the mind, who weaves a dreamscape around Bo and convinces her that the world would be better off without her. Just as Bo is about to end her own life at his bidding, a voice - Kenzi's - breaks through the reverie and snaps Bo back to her senses. She quickly dispatches her pasty foe and declares allegiance to neither Light nor Dark - she will spend her days among humans. The Morrigan tries to persuade The Ash that they should kill her, but both are convinced by an outside Fae called Trick that if someone has been able to keep Bo a secret for this long, it must be for a reason, and they should get to the bottom of the subterfuge before killing the girl.
Bo is released, along with Kenzi, to live as normal a life as she can, but she's warned to stay where the clans can keep an eye on her. The newfound Fae is relieved to know that there are others like her in this world, and also aware that this knowledge itself is the beginning of a very long road.