When Sienna Hegarty, a troubled friend of his daughter, comes to Joe O'Loughlin's door terrorized, incoherent, and covered in blood, he vows to unearth the dark secrets her mind has buried. The police find a major piece of the puzzle at Sienna's house: her father, a retired cop, murdered. Tests confirm that it's Mr. Herharty's blood on Sienna. She says she remembers nothing. 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.