 " Underneath " was written and directed by executive producer John Shiban . This marked his directorial debut . According to Shiban , part of the inspiration for the episode was that the series production staff had " actually talked for some time about doing a Jekyll / Hyde story but never quite found a way to do it " until the idea to use DNA came into play . Another inspiration for the episode was the 1949 film The Third Man , which featured a climactic chase through a sewage system . 
