The four-series, four-episode Superhero Crossover, featuring DC Comics characters from Supergirl, The Flash and Arrow, concludes tonight on this series – with all the feel of one of those old-fashioned spinoff story lines that would force kids, a generation or two ago, to buy different comics and sample different comic books, just to follow one complete story over a range of issues and titles. Marvel Comics is doing it with their TV shows and movies, so why not DC?