A teenage girl --- Sienna, a troubled friend of his daughter --- comes to Joe O'Loughlin's door one night. She is terrorized, incoherent --- and covered in blood.
The police find Sienna's father, a celebrated former cop, murdered in the home he shared with Sienna. Tests confirm that it's his blood on Sienna. She says she remembers nothing.
Joe O'Loughlin is a psychologist with troubles of his own. His marriage is coming to an end and his daughter will barely speak to him. He tries to help Sienna, hoping that if he succeeds it will win back his daughter's affection. But Sienna is unreachable, unable to mourn her father's death or to explain it.
Investigators take aim at Sienna. O'Loughlin senses something different is happening, something subterranean and terrifying to Sienna. It may be something in her mind. Or it may be something real. Someone real. Someone capable of the most grim and gruesome murder, and willing to kill again if anyone gets too close.