In recent years, Masterpiece Theatre productions have typically taken place in the 1900s with a particular fascination with WWII and its many-faceted complications and great depth of sorrows. This is why on the eve of ITV’s phenomenal Downton Abbey playing its swan song, PBS introduces us to our new addiction with a new time period. Inspired by the stoic lives of the volunteer nurses and doctors of the Civil War, this is Mercy Street.