Part mystery, part hospital soap opera, and part social commentary, Mary Roberts Rinehart's 1914 novel, "K., " is a look at a vanished world, a social artifact from the early 20th century, complete with rowdy, old-school roadhouses, forlorn lovers, and themes of revenge, altruism, and pride, not to mention a mysterious stranger known only as K. Recent dramatic series, like "The Knick, Mr. Selfridge, " and "Downton Abbey, " have sought to give us a clear-eyed look at things as they were a century ago, but "K." is the real McCoy, a time capsule taking us directly into that period and showing us life as it was before the Great War, before penicillin, before the vast changes in social norms that we now take for granted. One of the first romantic mysteries, it is also a window in on medicine as it was practiced a hundred years ago, and, as such, provides the perfect backdrop to delve into the mysteries of the human heart as well as the mysteries of our own past.