The old switched-at-birth premise may be familiar, but the recipe gets a few new ingredients in the French film “The Other Son.”

Writer/director Lorraine Levy twists the formula by following two 18-year-old boys, one of whom is Palestinian while the other is Israeli.

Joseph (Jules Sitruk) is preparing to begin his mandatory military service when a blood test reveals that he could not possibly be his parents’ son. It turns out his biological mother and father live in the West Bank and, after a hospital mixup, have raised Yacine (Mehdi Dehbi) as their own.

Naturally, this information unleashes a cascade of competing emotions for both families. Yacine’s brother, Bilal, has perhaps the most predictable reaction. He turns angry, hardly ready to soften his anti-Israeli sentiments upon learning that his best friend and roommate is Jewish.

The other responses are far more subdued (with the exception of Joseph’s sister, who provides one of the few light moments when she wonders “Will we have to give him back?”). The understated approach is probably a smart one, even if it doesn’t provide surprises.

Joseph’s and Yacine’s fathers have equally brooding temperaments and similar reactions to the news — one aggressively washes his car while the other heads under the chassis of his own automobile. And both mothers react with the same sweet sadness, hoping to bond with their biological sons while making clear they still love their unwitting adoptees.

The little sisters of the boys find common ground through dolls, of course. Even Yacine and Joseph have echoing open-minded temperaments, given that Yacine grew up in Paris and Joseph is a mellow, pot-smoking aspiring musician.

Levy works hard to lay out her humanistic stance — See? We’re all the same! — but it feels a little simplistic given how deep-seated and persistent the Palestine-Israel conflict is.

More often than not, the emotional depth of the film comes from the actors more than the material, and Areen Omari is particularly affecting as Yacine’s mother, who receives the news about her son with a memorable mix of confusion and inconsolable anguish. At that moment it’s easy to forget about the far-fetched and routine premise. The plot becomes an invitation for contemplation.

(At the Glenwood Arts.)