When director Richard Linklater's Before Sunrise hit theaters in 1995, there was no indication that it was just the first chapter in a series that would grow into the most important cinematic love story of all time. Sure, it was sweeter, smarter, and talkier than the average movie love story. But Before Sunrise didn't seem to promise anything beyond its low-key charms and the fresh-faced appeal of stars Ethan Hawke and Julie Delpy. When it opened that January — opposite Highlander 3: The Final DimensionBefore Sunrise earned appreciative reviews. But it certainly didn't seem like the kind of film that could spawn a series.

The ending of Before Sunrise was a kind of cynicism test for each viewer: Either you believed that young lovers Jesse and Celine really would reunite after six months had passed, or you thought they were naive kids who would move on with their lives. And for nine years, that ending remained an open question. Then Before Sunset hit theaters in the summer of 2004.

As it turned out, Before Sunset cleverly managed to have it both ways: Jesse had returned to Vienna as he'd promised, but Celine hadn't because of the sudden death of her grandmother. Instead, Jesse and Celine reunite in Paris several years later, as Jesse concludes a promotional tour for a book he's written. "Is it autobiographical?" asks a fan in the film's opening scene. "Isn't everything autobiographical?" he dodges. But we know better; his novel tells a suspiciously familiar story of a single, magical evening his protagonist spent with a woman in Vienna. And then Celine walks into the bookstore, and back into his life. His novel suddenly needs a new ending.

Before Sunset is a masterpiece — and one of the past decade's most unlikely and brilliant sequels. It's a story that unfolds in real time, as both Jesse and Celine let down their guards and reveal how haunted they are by their missed connection nine years earlier. "God, why didn't we exchange phone numbers and stuff? Why didn't we do that?" says Jesse. "Because we were young and stupid," says Celine. "I guess when you're young, you just believe there'll be many people with whom you'll connect with. Later in life, you realize it only happens a few times."

Back home in America, Jesse feels stuck in a loveless marriage, though he's enormously devoted to his son. Will he give up his unhappy life to be with Celine? Before Sunset ends, like its predecessor, with what seemed to be a cliff-hanger. While hanging out in Celine's apartment, she casually notes that Jesse is going to miss his plane, leaving audiences to decide for themselves whether or not he'll leave her. But watch the scene again. Look at his face. There's no cliff-hanger. He's not going anywhere.

So now, with Before Midnight, which opens in limited release today, it's nine years later — in both the movie's timeline and in real time. Jesse and Celine have gotten together — and the real surprise is how complicated their lives have become after the easy chemistry of the first two films.

Before Sunrise and Before Sunset are essentially showcases for Hawke and Delpy, with other characters only occasionally popping into their intensely romantic bubble. But Before Midnight doesn't open on Jesse and Celine; it opens on Jesse and his son Hank, who sports the same scruffy Converse sneakers favored by his father in Before Sunrise, as Jesse reluctantly puts him back on a plane to America. After giving us the satisfaction of knowing that the longtime star-crossed lovers are finally together, the first act of Before Midnight largely separates Jesse and Celine. At a communal dinner they share in Greece, we even hear the love stories of other characters — including a younger couple, who would be in exactly the same position as Jesse and Celine in Before Sunrise if not for Facebook.

During Before Midnight's languorous opening scenes, we're invited to think about the 18 years that have passed in this story — and, by extension, the 18 years that have passed in the actors' lives. Hawke and Delpy were both in their mid-20s when Before Sunrise was filmed, and they're both over 40 now. It's hard to overstate the effectiveness of this approach; film series almost never last long enough to see characters get older, and when they do — think Sylvester Stallone in Rocky Balboa, or Leonard Nimoy in the new Star Trek films — those appearances hinge on a nostalgic appreciation for their performances of the past.

After the romantic travails of the previous films, I don't think anyone would have objected if Before Midnight spent an hour and a half with Jesse and Celine as they wandered around Greece, talking about how blissfully in love they are. But the movie aims so much higher than that. Nine years is a long time to be apart. But it's also a long time to be together, and the intensity of Jesse and Celine's earlier encounters has evolved — or, perhaps, devolved — into the low-watt comfort they display in Before Midnight.

In the margins of the film, we get hints about what we've missed between Before Sunset and Before Midnight. Jesse and Celine have raised two adorable twin daughters, and have custody of Jesse's son every summer. Jesse's ex-wife hates Jesse and Celine. (And despite our loyalties to these characters, it's kind of hard to blame her.) Jesse even wrote a sequel to his first novel based on the events of Before Sunset.

But the familiarity Jesse and Celine amassed over the past nine years has also bred some contempt. In an early and telling scene, Jesse — so moony and romantic in the first two films — misses a clear opportunity to make Celine feel special when she asks if he'd pick her up on a train again. "I wanted you to say something romantic and you blew it," says Celine. "You failed a test, and the truth is, you would not pick me up on a train. You would not even notice me — a fat-ass, middle-aged mom, losing her hair…"

By the time Jesse and Celine reach the hotel where they're supposed to spend the night together, the small hurts they've amassed have curdled into something uglier, culminating in one of the most realistic and painful fights in film history. "I know you better than I know anybody else on the planet, but maybe that's not saying much," says Jesse. But these two know exactly how to hurt one another. It's shocking, as viewers, to learn that the couple that was so easy to romanticize in the first two films has real (and potentially existential) problems. It's easy to sympathize with both of them simultaneously. Neither Jesse nor Celine denies cheating over the course of their nine-year relationship, and when Celine storms out, it seems like their relationship might be over for good. By the end of the movie — which mercifully segues back into the romanticism that characterizes the first two films — they've reached a truce, but it's an uneasy one. We're used to seeing this couple fighting through their external problems, but their internal ones are far more difficult to overcome, and far harder to watch.

Does that sound unpleasant? It is. Hollywood tends to focus on romance when it's in full bloom, avoiding the years of difficult, day-to-day work that challenge even the happiest and healthiest of couples. Director Richard Linklater and Ethan Hawke have joked about Michael Haneke's devastating Alzheimer's drama Amour, which was nominated for Best Picture last year, being the logical endpoint for Jesse and Celine's story. But jokes aside, that's not that far off. Relationships can be unpleasant, and there's a reason that the vast majority of them end. No matter how much you love someone, you can't be together without mutual sacrifice, and deciding to stay together isn't a decision you make once; it's a decision you make every day, for the rest of your lives together.

That's the message that Before Midnight — painfully, challengingly, and thrillingly — understands. The most important line in the movie comes in the middle of that brutal fight, as Jesse tries, unsuccessfully, to stop the argument in its tracks before it's too late. "I am giving you my whole life," says Jesse. "I've got nothing larger to give."

What Before Midnight understands about relationships — and what virtually no other mainstream film is willing to acknowledge — is that getting together is the easy part. Staying together is hard.

In the vast majority of cinematic romances (including many of the films routinely cited as the best love stories of all time), we meet the couples at the height of their romance, and leave them soon after they've finally gotten together. But Before Midnight tackles Jesse and Celine's relationship nine years after Before Sunset's absurdly romantic reunion, showing relationships as they really are — and there's a good chance that their story still isn't over yet.

The end of Before Midnight is a far bigger cliff-hanger than the end of Before Sunset. Jesse and Celine make it out of their argument in Before Midnight with their relationship intact. But nine years from now, will the fourth film in the series open with Jesse and Celine apart once again — and this time by choice?

"I assure you, that guy you vaguely remember — the sweet, romantic one you met on the train? That is me," insists Jesse as the film comes to a close. But can Celine accept the rest of him, and can he accept the rest of her? That's a real love story — the kind that Hollywood never tells, and the kind that's the most important.