A double life, a privileged life. Obsession and sexual fantasy. Who knew his secret? Was there no one to stop him? 48 Hours Mystery examines the double life of Boston University medical student Philip Markoff, as well as the life of his alleged victim. With the help of private investigator Joe Moura, we learn more about the investigation and the charges against Markoff. Boston Globe reporter Maria Cramer, forensic psychiatrist Dr. Michael Welner and a forensic computer expert add their insight in the case. We look at the dark side of the digital world, and the expanding world of Craigslist. Has it become the new portal of prostitution? Our shocking investigation exposes a Web of complicated relationships. And we take a look at the life of Julissa Brisman, 25, whose young life was cut short as she worked to get it back on track.