“Didn’t we meet at Marienbad last year?” an unnamed man asks an unnamed woman. So begins one of the most confusing (and frustrating) movies ever made. Two people are at a French château. One says they know each other. The other denies it. Back and forth they go, over and over, while director Alain Resnais’ camera endlessly stalks the château’s spooky corridors. All the while, organ music blasts out at unpredictable moments. Some well-dressed people stand in a sunlit garden. They cast shadows. The triangle-shaped trees surrounding them do not. There appears to be no plot. It’s all very dreamlike. What is this movie about?