Three years ago, fifteen-year-old Ellen Vicksburg went missing from the quiet town of River Remez, New Mexico. She was kind, studious, and universally liked. Her younger brother Ben could imagine nothing worse than never knowing what happened to her, until he found her body in a shallow grave by the river's edge. Ben is determined to find the monster who abducted and strangled Ellen. Police believe she was the victim of a psychopath known as the Demon. But Ben--a math geek too smart for his high-school classes--continues to pore over the evidence at the local police precinct. In his sister's files, Ben's analytical mind sees patterns that don't fit, tiny threads that he adds to the clues from other similar unsolved murders. As the body count rises, a picture emerges of an adversary who is as cunning and methodical as he is twisted. At first the police view Ben's investigation with suspicion. Soon his obsession will mark him as a threat and uncovering the truth may not be enough to keep Ben and those he loves safe from a relentless killer who has nothing left to lose.