In 1964, 19-year-old Mary Sullivan was strangled to death in her apartment, the 13th victim of a killing spree that had terrorized Boston for 18 months. Her death remained unsolved for nearly 50 years. The same is true for a dozen more deaths attributed to the Boston Strangler. In our first episode, we meet Mary, as well as the killer’s first victims, and begin to explore the many unanswered questions that continue to haunt the victims’ families, the police who worked on the case, and the reporters who turned it into a national story.