What’s so seductive about the “Before” movies, which consist of “Before Sunrise” (1995), “Before Sunset” (2004) and the upcoming “Before Midnight,” is the unique concept of following two very flawed but likable and engaging people and watching how their unorthodox relationship changes every nine years. By now, it would perhaps be more devastating for us to part with Jesse (Ethan Hawke) and Celine (Julie Delpy) than it would be to watch them part ways. Part of what’s made these movies so personal is that the audience is just as entwined in this messy, complicated relationship as Jesse and Celine are. We’re in it together. We’re too involved at this point.

Director Richard Linklater’s little romance came out of nowhere in 1995 with a really surprising, really smart pairing of two actors who were out to prove themselves. It wasn’t like “Before Sunrise” didn’t know that Jesse and Celine were flawed. Yet the movie was so calm and humane that we couldn’t help but fall in love with them, and Linklater had so smartly incorporated Vienna into the film that the romance could have only felt overflowing.

Nine years later, the unexpected “Before Sunset” arrived. This time, we were witnessing more than just a playful flirtation between two young people. The characters didn’t feel so storybook this time — despite how picturesque Paris felt as a backdrop to their walk-and-talk. We watch Celine really turn into a mess, threatening Jesse that she’d hop out of the cab in tears when he doesn’t remember they’d made love not once but twice. And Jesse confides just how imperfect his life has turned out; a crumbling marriage with a kid complicates matters even more. “Before Sunset” was a tiny interval where Jesse and Celine are briefly reacquainted, but we knew they were going to have to part again. However, we left them on a promising note, pondering the possibility that Jesse might just miss his flight and stay with Celine.

Eighteen years later, we are reunited once again with Jesse and Celine in “Before Midnight.” What surprises and amazes so much about “Before Midnight” is just how profoundly Jesse and Celine have changed over the years. They’re still the same people, but now we sense something heavier weighing on them. Linklater has framed this movie very differently than he did the first two; for the first time, we see third-party individuals interact with and influence Jesse and Celine in significant ways and for large parts of the movie.

When they eventually embark on that trademark walk-and-talk we so yearn to see them go on, it’s not the same; they’re more exhausted and less buoyant. What’s changed is that, at last, Jesse and Celine decided to leave behind their storybook romance and have finally committed to each other. Linklater, Hawke and Delpy have aptly removed the youthful dreaminess that so perfectly enveloped the first two films and have replaced it with something stronger and deeper: reality. It’s ironic that now that Jesse and Celine are finally together, the fate of their relationship feels more precarious than ever before. The dynamic of their relationship is too real and too common now that we have a stronger inkling this time of where exactly this partnership might go. For the entire movie, we worry for them and agonize with them, but that’s what makes “Before Midnight” so beautiful. We know them too well by now, so we ache knowing how connected we are to them and how much their decisions will affect us. And they do. “Before Midnight” is the richest but also the saddest of all the movies.