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.