When Asghar Farhadi's "A Separation" begins, a middle-class husband and wife in Tehran are arguing about her petition for divorce in front of an unseen judge. For a while the judge indulges their anger, which is compounded by hurt. "My finding," he finally says, "is that your problem is a small problem. Please sign here and good day." The judge is no fool. Neither is anyone else in this contemporary masterpiece from Iran; everyone has their reasons. But the couple's problem isn't small, at least not for them, and their ensuing separation opens up a succession of fissures, chasms and moral dilemmas that define an entire society.

Thematic sweep and dramatic specificity are the movie's hallmarks, with the one always growing out of the other. The marriage is beset by fatefully conflicting circumstances. Simin, the wife, wants the family to seek a better life abroad for the sake of their 11-year-old daughter, Termeh. Nader, the husband, refuses to leave Iran; he's taking care of his father, who's impaired by Alzheimer's, and won't consent to Termeh living elsewhere with her mother. Soon the dire requirements of home care force Nader to hire a caregiver, Razieh, who comes to the task with her impish little daughter, Somayeh, in tow, and with religious strictures that contribute to a terrifying series of events in which the fabric of modernity is torn asunder and the line between truth and falsehood shifts and blurs.

(Before naming names, let me say that the members of the cast represent ensemble, naturalistic acting at its finest. They include Leila Hatami as Simin; Peyman Maadi as Nader; Sarina Farhadi—she's the filmmaker's daughter—as Termeh; Sareh Bayat as Razieh; Ali-Asghar Shahbazi as Nader's father; Kimia Hosseini as Razieh's daughter; and Shahab Hosseini as Razieh's husband.)

When admirers asked Mack Sennett how he went about creating his classic silent comedies, he would describe the basic principle as "One thing leads to another." Far from being a comedy, "A Separation" is an enthralling drama—with some kinship to "Kramer vs. Kramer"—and the subtitled Persian dialogue is fluent and copious. All the same, one thing leads to another with such ease and inexorable logic that the script could have been created by the filmmaker taking dictation from the people on screen. And as long as I've invoked the name of Mack Sennett, I should also note that everyone having their reasons is a maxim most famously enunciated by Jean Renoir, who might have recognized his own spaciousness of spirit in the film's generosity toward its conflicting and conflicted characters.

Simin has beauty on her side, though not always unvarnished truth when she's working her marital stratagems. Nader comes on as a hard case, slow to smile and quick to criticize; only gradually does he reveal warmth and humor, along with a willingness to fudge crucial facts when coming clean might court disaster that he doesn't deserve. The caregiver, Razieh, is a picture of piety—literally so, since she has the face of a Madonna—but the secrets she chooses to keep may destroy lives. The daughter, Termeh, watches and listens while seeking her own truth in what ultimately becomes a chastening story of her coming of age. Razieh's husband, Hodjat, emerges as the film's most frightening presence and yet, at one and the same time, its most compelling figure. He comes on as a religious fanatic, but what we learn about him illuminates, ever so affectingly, the fundamental separation in today's Iran between the secular bourgeois class and the religious working class.

"A Separation" reaches American screens—and joins the Oscar race as Iran's official entry—at a time of escalating tension between the two nations. What's more, one of Iran's most prominent—and provocative—filmmakers, Jafar Panahi, is currently in prison after the Iranian government sentenced him to six years of incarceration and a 20-year ban on any activities connected with making movies. Why, then, did the authorities put their stamp of approval on Mr. Farhadi's film? He was asked that question at last summer's Telluride Film Festival. "Censorship in a country like mine," he replied, "is a little bit like autumn weather. There may be rain in the morning, then sun later. I can only guess that my film came before the censors when the sun was shining. It may also have helped that I haven't judged anyone in it."

The latter point is essentially true, though he does pass one judgment in a way that may have escaped the censors' attention (or, for all we know about the internecine workings of a secretive government, may have been noted with tacit approval). In the film's intertwined wars—between the couples as well as the spouses—superior wisdom is clearly judged to lie not with either of the rigid, prideful husbands but with Nader's wife, Simin, who makes an impassioned plea for the peaceful resolution of both conflicts. Does that constitute an oblique comment on the state of affairs between two hostile states? Maybe yes, maybe no, but the main thing about Mr. Farhadi's film is vivid humanity. "A Separation" is its own shining light.