A Certain Smile is narrated by free-spirited young law student Dominique. She has a boyfriend, Bertrand, and they romp around happily enough, even if she doesn’t feel particularly passionately about him, rationalizing:

Trust, esteem and tenderness were not to be despised, and I thought very little about passion. The absence of genuine feeling seemed to me the most normal way to live.

Bertrand foolishly introduces her to his married uncle, Luc — and Dominique recognizes immediately what Bertrand has fatally overlooked:

He’s just the kind that seduces little girls like me.

The formulation is revealing: if Dominique behaves like an adult and acts very independently, she nevertheless recognizes that in matters of passion she’s an immature child. And, indeed, despite recognizing what Luc is capable of, she falls for him hook, line, and sinker.
Luc admits his devotion to his wife, but that doesn’t mean he doesn’t like to have some fun on the side — and he easily convinces Dominique to go along with it. From the first, however, he also reminds her:

In this kind of thing nothing matters too much. I like you. I love you. We’ll have fun together. Nothing more than fun.

He promises the epitome of a casual affair — and he reminds her later on again:

It’s not very serious with you, either. Nothing is very serious.

But, of course, the problem with passion is that one does take it rather seriously. Dominique repeatedly reminds herself of where this is going — nowhere — but the appeal of their life in the moment is enough for her to keep pushing the inevitable out of mind.

Shortly, we’ll have dinner, then we’ll sleep together, and in three days we’ll say good-by. He’ll probably never again be the way he is now. But this moment is here; it’s ours. I don’t know whether it’s love or understanding, but that doesn’t matter. We’re alone, each one on his own.

Of course, Dominique’s solitariness is also shaped by her choices; men are readily available to her — she has numerous encounters with the eager and willing — and Bertrand certainly gave her enough opportunities to enjoy a more traditional, coupled happiness, but she prefers to remain flighty (making her moans about being: “Alone. Alone.” feel rather rich). No doubt, part of the appeal — at least initially, and presumably also subconsciously all along — of an affair with Luc is that she knows it can not be lasting. At her age, perhaps not the worst of life-choices — except, of course, that it gets to her, and that she does feel loss and regret.
After a two-week-getaway with Luc which he refers to as a period of “cohabitation” Dominique blurts out:

“This wasn’t cohabitation,” I protested laughingly. “It was a honeymoon.”

She may try to be lighthearted about it, but of course deep down this is what she wishes for — that this was a beginning, rather than merely an episode. And the beginning of something domestic and traditional at that.

“It was a simple story”, Dominique sighs in closing, and A Certain Smile certainly is a simple story — but, of course, affairs of this (and most every) sort are also infinitely complex. Sagan does a decent job of presenting this girl and her affair and how she thinks about it. Still, it is all a bit thin and juvenile — but then so is Dominique (and so was author Sagan, at the time) — and it reads just like what it is: the worldly-wise (i.e. not so …) thoughts of a barely twenty-one year old girl whose romantic fantasies are at odds with her physical indulgences.

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