It's yet another madcap British comedy, as class conflict, sexual desire, and mistaken identity give rise to—literally—years of wacky repercussions.
Oh, wait. Not a comedy? Well, nuts. That changes my whole interpretation of the movie.
It takes place in 1935, 1940, and (roughly) the present. The 1935 section is interminable, carefully—too carefully—setting up characters and motivations for the rest of the film. But fortunately, the main characters wind up three-dimensional: people I had thought to be simple upperclass twits turn out to have depth and redeeming qualities.
The scenes set around pre-evacuation 1940 Dunkirk are jaw-droppingly good, catching the filth, chaos, and confused desperation of the British soldiers waiting for a ride across the Channel. If you need a reminder that war is Hell, this is a pretty good choice.
The brief scene set in the present is either (depending on your point of view) a neat little trick, or a dishonest little cheat. I'm ambivalent.
And it did get nominated for 7 Oscars, including Best Picture.