When a 15-year-old girl vanishes on her way home from a friend’s quinceañera rehearsal in 1998, a California community holds its breath and hopes it’s a one-off. But when other young women in the community are targeted one by one, investigators have to ask if there’s a serial killer in their midst.

Thirty-three years after Douglas Wagg, Jr. turned up on a lone stretch of railroad tracks in the middle of the night in rural Martin County and over a year since Delia took on the case the scope of what was really going on in the area during the 1990's has come into view. Who was Doug? How did he...