Three years ago, sixteen-year-old Ellen Vicksburg went missing from the quiet town of River Remez, New Mexico. Ellen was kind, studious, and universally liked. Her younger brother, Ben, could imagine nothing worse than never knowing what happened to her - until, on the first anniversary of her death, he found her body in a deep grave by the river's edge. Ben, now seventeen, is committed to finding the monster who abducted and strangled Ellen. Police believe she was a victim of a psychopath know as the Demon. But Ben - a math geek too smark for his high-school classes - continues to pore over the evidence at the local police precinct, gaining an unlikely ally in his school's popular new girl, Ro Majors. In his sister's files, Ben's analytical mind sees patterns that don't fit, tiny threads that he connects to clues form 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. But 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.