The title of Stephan Krohmer’s “Summer ’04,” which opens today at Film Forum, sounds nostalgic, as if evoking a moment from a generation ago instead of a few years past. The bourgeois family in this unsettling film spends its vacation in an airy cottage that is a ready-made memory, with doors flung open to breezes and sunlight. The idyllic setting is so relaxing, and the family’s manner so laissez-faire, that it all feels destined to curdle within minutes.

A suitably louche transgression occurs, but what’s striking and even a little disorienting about this German drama is how little Mr. Krohmer judges his characters. And since they in turn hardly think to question themselves too deeply, even when edging into dubious behavior, you don’t know whether to admire the film’s depictions for their honesty or refuse to believe out of faint distaste.

Like many American suburban dramas, the tension in “Summer ’04” hovers around underage sexuality. Miriam (Marina Gedeck) and André (Peter Davor) are worried about their teenage son’s precocious girlfriend, Livia (Svea Lohde), who is staying with the family while her parents holiday in Mexico. The two banter about whether the self-possessed 12-year-old is too young for their son Nils, but then they notice that the youngsters don’t seem too close, anyway. To his father’s consternation, Nils even lets Livia abscond with the family boat to go sailing with someone else.

That someone else turns out to be Bill (Robert Seeliger), a German-speaking American expatriate in his 30s living off a comfy inheritance. Accompanying his much younger companion home, he comes off as polite but a little too nice, and André peevishly waves off his compliments as a gauche American habit. But throughout the encounter, his wife wears a delighted smile, reflecting her amusement but also telegraphing an interest in the slightly oafish but hunky foreigner.

Miriam’s attraction to Bill and responsibility for Livia form the unnerving axis of the ripening plot, shadowed at first by the American’s clouded motives. When young Livia stays out too late at Bill’s house and no one answers the phone, Miriam is worried (or annoyed) enough to drive out to confront him. He responds with clumsy, vaguely creepy evasions, and she’s spooked enough to phone her husband and tell him to get help if he doesn’t hear from her.

But in the ensuing week, Miriam contrives to sail with Bill, and she makes the first move on a secluded lake bank. As their affair unfolds, Mr. Krohmer’s evenly toned film avoids the expected sense of scandal or liberation; even the fact that Miriam is competing with a pre-teenager yields unexpectedly complex ironies. Unlike the “Moral Tales” of the French filmmaker Eric Rohmer — to which Mr. Krohmer’s film has been compared — the dialogue refuses to elucidate (and thereby dignify) the situation.

“Summer ’04,” which made its premiere at Cannes last year, beguiles with a hard-to-place resilience and an unpredictable flow that detours after a tragedy is greeted with damning under-reaction. Vital to delivering this tricky material is the lively presence of Ms. Gedeck, who most recently starred in “The Lives of Others” (coincidentally, in a love triangle). Mr. Davor’s smaller turn as her husband is also critical, as he embodies the casual arrogance that underscores the couple’s attitudes and seems to have led them into confusion in the first place.

Yet the film also feels like a journey without a compass, and at some level, I’m not sure I entirely buy it. The characters’ self-assurance comes to feel like a catch-all, covering what is actually an unwillingness to explain what’s really going on inside them — especially Livia, who remains tantalizingly underdeveloped. If “Summer ’04” is like the memory of a transformative period in a family’s life, it needs a bit more of the reflection that the characters are unwilling to provide themselves.

Through August 14 (209 W. Houston St., between Sixth Avenue and Varick Street, 212-727-8110).