When a teen falls while taking a selfie at the edge of a cliff, the last thing she sees before plummeting to her death is Katie Harmon, the nine-year-old girl she was babysitting, looking down at her.

Investigators gather at the scene, and Katie's mother, Sara, rushes to comfort her daughter. Yet there's a small, secret ping of alarm in Sara's heart that she cannot share--though rookie Detective Kim Pierce senses it.

For years, others have tried to unravel this secret. From true-crime podcasters to a haunted journalist searching for a killer who vanished after being released from prison several years ago. And now, with detectives tightening the focus of their investigation, Sara is consumed by her darkest fear--that the babysitter's death was not an accident.