The film Dunkirk tells a very important story. During the Second World War the British, French and other allied forces get surrounded at Dunkirk, a beach town in France. The limited Navy and Red-cross ships can't seem to make it back across the channel without being hit by German forces, and British fighter planes don't have the fuel capacity to be of much help. Overall, the situation is very grim. The British Navy commissions the use of any serviceable ship or boat to rescue the 300,000 some odd soldiers trapped at Dunkirk.
Now you are prepared to watch a movie that has virtually no commentary or character development. Dunkirk is well done, but you need to know the basic story before you watch it, and even then you may still have some questions.