13-year-old Mary Day disappeared in 1981. More than 20 years later, investigators suspected she’d been murdered, and focused on Mary’s mother and stepfather, who had never filed a missing persons report. But just as investigators seemed to be closing in on the truth, the case would take a shocking turn. "48 Hours" correspondent Maureen Maher reports. This classic "48 Hours" episode last aired on 12/23/2023. Watch all-new episodes of “48 Hours” on Saturdays and stream on demand on Paramount+.