Richard Linklater's 1995 film Before Sunrise and its first sequel, 2004's Before Sunset, tell a pair of simple stories. In the first, a young American named Jesse (Ethan Hawke) strikes up on a conversation on a train with a pretty French girl named Celine (Julie Delpy). There's a spark, and on the spur of the moment, he makes a suggestion: that she get off the train with him and spend the night walking and talking in Vienna. Intrigued, she takes him up on the rather risky invitation, and over the course of that night, they fall into something resembling love. In the second film, the couple reconnects nine years later, as Jesse (now an unhappily married father) spends the last few hours of his European book tour—he wrote a novel based on their initial encounter—catching up with Celine in Paris. That film ends with the hint that he might make a choice as daring as hers at the beginning of the first: to "miss that plane," and hit the reset button on his entire existence.
In other words, the first two films were about the reckless impulsiveness of young romance. Ingeniously, the third film in the series, Before Midnight, is about the consequences of that impulsiveness.
A word of warning: If you've somehow managed to avoid learning where Jesse and Celine have landed, by all means, preserve that surprise. The 2004 film managed to keep the narrative specifics under wraps, but that was a different time in movie marketing, and since the trailers and promotional materials for this one aren't keeping the secret, it's apparently up for discussion. So here's what's what: Jesse did, in fact, miss that plane. He and his wife split, he and Celine have cohabitated but not wed, and they have a pair of twin girls together. They live in Paris, so Jesse doesn't see his son from his previous marriage as much as he'd like, and as the film begins, he's putting the boy on a plane for the States at the end of a family vacation in Greece.