Lovely. Insightful. Sad. Funny. “Before Midnight,” the third chapter in the continuing love affair of expatriate Jesse (Ethan Hawke) and Parisian Celine (Julie Delpy), is essentially a couple walking and talking. But what a pair, and what dialogue!

The actors, returning to the roles they originated 18 years ago, speak their own lines, which they wrote in collaboration with the series’ director, Richard Linklater.

They have internalized their characters’ bloodlines in “Before Sunrise” (1995) and “Before Sunset” (2004). They function not as ordinary co-stars but acting partners. We follow sassy Celine and emotionally conflicted Jesse for a day as they wind down a family vacation in an idyllic Greek resort. The morning’s talk is casual, teasing and affectionate; they are still in love. But even in Eden, trouble is around the next tree.

The film opens with a poignant airport scene as Jesse sends his visiting 13-year-old son, Hank (Seamus Davey-Fitzpatrick), home to Chicago and the care of his time-bomb ex-wife. It’s an affectionate, awkward farewell. Jesse, once a footloose Gen-Xer, worries that he has missed the most important years of the boy’s life. Hank warns Jesse that attending his music recital would cause a lot of grief back home. Through this prelude, the kid has a backpack slung over his shoulder, an understated image of the baggage he’s carrying.

Then comes a 14-minute, single-camera take of Jesse and Celine’s car chat, catching us up on their last nine years as a couple. Celine, too, frets that she’s losing her best years. Her eco-engineering business is floundering, her hips are thickening, her hair is thinning and the couple’s angelic twin daughters are at a needy age.

She pushes Jesse to say whether he still finds her as alluring as he did when they met and fell in love on a Vienna-bound train at 23. “Right now as I am. Would you start talking to me?”

“You’re asking a theoretical question. I mean, what would my life situation be? Technically, wouldn’t I be cheating on you?”

“I wanted you to say something romantic, and you blew it!”

As the hours wear on, an undercurrent of frustration emerges. Over a lunch of wine and rich conversation with friends, the bourgeois niceties of their life fray thread by thread. There is a terrible evening of mutual recrimination and self-justification. Imagine the warm, free-ranging jazz melodies of Dave Brubeck descending into Miles Davis dissonance and you have the flow of Act I and Act II. The witching-hour resolution, taking a glimpse into the future, is a terrifically complex and deep questioning of life’s meaning and the value of love, loyalty, friendship and family.

You don’t need to have seen the earlier episodes in Jesse and Celine’s adventures to understand and enjoy this one. You may find yourself seeking them out and watching them anyway. “Before Midnight” is sublime as a stand-alone, as the finale of a trilogy that brings everything full circle, or as a milepost in a captivating continuing story. At the fadeout, I was hoping this was not “goodbye” but “au revoir.”