Before Midnight. Ethan Hawke, Julie Delpy. Directed by Richard Linklater. Rated R. Opens Friday.

When Before Sunset appeared nine years ago—and also nine years after the previous single-day adventure of lovers Jesse (Ethan Hawke) and Céline (Julie Delpy)—it seemed like a miracle. Nobody had exactly been clamoring for a sequel to Before Sunrise, yet director Richard Linklater and his two stars built upon the tentative, first-blush emotions conjured up by the original and created something deeper, thornier and yet still swooningly romantic. Are they pushing their luck by trying a third time? Happily, no. Before Midnight was celebrated by many at Sundance, where it premiered, as the series’ masterpiece; they overstated the case, but there’s no question that the film earns its place in the most remarkable long-term cinematic project since François Truffaut introduced his alter ego Antoine Doinel with The 400 Blows way back in 1959.

If you’ve become heavily invested in Jesse and Céline as fractiously flirtatious ideals, however, be prepared for a slight jolt. (Those who’d prefer to experience that jolt firsthand are encouraged to stop reading now and return to the review later.) In the nine years since the last installment, the two have all but gotten married; they’re introduced here on a Greek vacation with their twin daughters, engaging in their usual lengthy, playful conversations ... but with the more combative edge of people who now know each other inside out, including every pushable button. Discussions of the nature of love and relationships with their Greek hosts lead to a climactic hotel-room meltdown that casts grave doubt on the prospect of film No. 4.

The trajectory is familiar; numerous movies, from Ingmar Bergman’s Scenes From a Marriage to the recent Blue Valentine, have tilled this soil before. But our emotional investment in these two characters (assuming you’ve seen Sunrise and Sunset, which is really a prerequisite), along with Hawke’s and Delpy’s masterful and brutally realistic expansion of their personalities over time, give it a kick that transcends routine. Arguably, this was the only direction the series could have taken (and remained compelling, anyway), and if it’s tough to watch these two journey from ecstasy to acrimony, it’s also uncommonly powerful. Will they still have anything to say to each other, or to us, in another nine years? I look forward to finding out.