400000 soldiers, British and French, stranded on a beach of Dunkirk, surrounded by German forces, wanting only to get the fuck out of there. While being bombed by Nazi planes. Fun stuff.
So, this is a big one. Made by Christopher Nolan, this is a war movie of this year, I would say, and rightly so. The Dunkirk evacuation started as absolutely hopeless thing, where they barely hoped to get back 10% of people, and in the end they did manage to return most of the soldiers home. The events historically show one of the biggest mistakes of Germany on western front.
Now, the question of this all being the truth is not even a question at all. The usual site says, that while stage is a real historical event (obviously), and it was represented pretty damn realistically, and even was filmed on the actual location, with a few actual boats that were used in the real event (which is pretty cool, really), the characters and their struggles are completely fictional. Movie centers on 3 semi-separate stories - soldiers trying to find any chance to get home, private boat owner and his son, sailing across the channel to help with evacuation, and fighter pilot providing cover for British ships from German bombers.
While all 3 stories are also based on actual events and behavior of different people at the time, specific details are very much made up. So it's a cinematic fiction, but the background for it is a very real event that was represented pretty accurately. All in all, good war movie, using historical event well for cinematic purposes.