The words of a 3-year-old child are not enough to convince investigators that an adult isn't telling the truth and a case goes unsolved. However, one steadfast detective believes in her and spends years working the case alone. As more evidence is brought to light and more alleged victims are found, the memories of the little girl prove to be true and a killer is brought to justice. NBC's Rob Stafford reports Friday at 9 p.m. ET, 8 p.m. CT on Dateline NBC.