All the President's Men (1976)

Directed by Alan J. Pakula

This is the fairly incredible behind the scenes story of how to reporters at The Washington Post broke the Watergate story (and the rest is history).  Here's the the thing about this movie though, it came out basically a year after the events of the Watergate scandal unfolded.  Because the scandal played out in headlines across country, pretty much everyone was very familiar with all the main players in the scandal.  This makes for the main fault of the movie today: If you didn't live through Watergate, read a book about it, or happen to be a poli-sci major, you really don't know who all these players are (and there are a lot of them) and the movie doesn't bother to explain it.  It's not so much the story of the Watergate scandal itself, but the story of the investigative journalism that lead to the story.