“Definitely, Maybe,” a nimble and winning little romance written and directed by Adam Brooks, begins with one of those awkward Important Talks that parents are sometimes required to have with their children. In this case Maya Hayes (Abigail Breslin) needs some debriefing after a sex education class at her Manhattan elementary school. She’s acquired some technical vocabulary but not a lot of context, and so it falls to her dad, Will (Ryan Reynolds), to do the necessary explaining.

This is a delicate task since in the grown-up world of love, theory and practice frequently part company, which is what Will and Maya’s mother have recently done. Like any parent Will wants to tell his daughter the truth without subjecting her to unnecessary disillusionment or cynicism. And Mr. Brooks faces a similar challenge, one that lesser romantic comedies tend to flee.

How do you preserve the fairy-tale elements of the genre — the beguiling fantasy of permanent bliss — in the face of certain prosaic and unavoidable facts? In the real world, after all, people divorce, sleep around, fall in love too soon, too late or too often.

It’s a mess, and a movie like this one has to acknowledge the mess without falling into it, in which case it would be a sad little melodrama rather than a sparkling comic enchantment. In spite of everything, viewers, like Maya, have to walk away with our faith in soul mates and happy endings confirmed, rather than compromised or shattered.

So Will decides to tell Maya a slightly edited, PG-13 version of the story of his life and loves before she was born. At first the ending seems predictable enough: after various false starts and digressions, he will finally meet and marry her mommy. But neither the girl nor the audience knows which of the women his younger self meets the mother will turn out to be. (And for a while we forget that, after their happy ending, Mom and Dad will end up divorced.)

The candidates are neatly — but not too neatly — sorted by hair color and temperament. Emily (Elizabeth Banks), the obliging, blond college sweetheart; Summer (Rachel Weisz), the intriguing, slightly dangerous dark-haired intellectual; and April (Isla Fisher), the impetuous redhead with whom Will trades insults at the New York campaign headquarters of Bill Clinton.

Ah, the ’90s. Among the many charms of “Definitely, Maybe” is the way it evokes the recent past without drowning in fussy period detail. Will, ambitious and idealistic, has come east from Madison, Wis. (leaving Emily behind), to plunge into the world of electoral politics; and while this movie is hardly an incisive political satire, it does capture some of the flavor of the times and, more generally, the headiness of youthful commitment to a cause.

Which is followed, perhaps inevitably, by disappointment. It is not hard to see that the Will of 1992 — smart, handsome, decent, happy in love, successful at work — is headed for a series of falls. Mr. Reynolds is dashing and sarcastic enough to fight off the lingering threat of blandness, and a subtle enough actor not to spoil his character’s unhappiness with self-pity. Will is, for all that, a bit of a cipher, or at least a sort of median Gen X-er, his edges smoothed away and his unruly appetites tamed.

Or maybe he’s just too modest and amiable to insist on being the most interesting person in his own story. Instead Mr. Reynolds is part of an attractive ensemble that includes Derek Luke, as Will’s best friend and sometime partner (they start a political consulting firm together after ’92), and Kevin Kline, as an aging gonzo journalist who is Will’s chief rival for Summer’s affection.

But it is the women in Will’s life — including Ms. Breslin — who make up the affectionate, unsentimental heart of the film. Ms. Fisher in particular enlivens every scene she’s in with a comic energy that’s antic and graceful.

“Darlin’ you can’t love three,” says an old folk song, and in movies of this kind the man who does is likely to be viewed as a cad. Either that, or the women he doesn’t end up with will be shown to have such egregious flaws that their rejection will come as a relief.

Emily, Summer and April are all decidedly imperfect, but Mr. Brooks succeeds in showing how their shortcomings are, especially at first, part of their allure. He also makes clear that Will, besotted with each in turn, does not know any of them as well as he thinks he does. He also is not quite sure what he wants. The treachery of love — and also its promise — is that people can surprise you.