For a picture that really isn’t much more than a high-toned soap opera, “The Other Woman,” originally called “Love and Other Impossible Pursuits”—the title of the novel by Ayelet Waldman on which the adaptation by writer-director Don Roos is based—is surprisingly effective. Much of the credit must go to hard-working Natalie Portman who plays Emilia, an ambitious young woman who parlays her post as assistant to married lawyer Jack (Scott Cohen) into an affair that breaks up his marriage to abrasive doctor Carolyn (Lisa Kudrow). Jack weds Emilia, but their relationship is tested by his young son William (Charlie Tahan), who’s not taking well to the changed family dynamic, and by the sudden death of Jack and Emilia’s infant daughter.

Roos tells the tale in a fractured way, beginning after Emilia and Jack get married and flashing back to their affair. And much of the action is devoted to the vacillating bond between the free-wheeling, rather irresponsible Emilia and precocious, emotionally fragile William—a relationship that the hostile Carolyn tries to poison at every opportunity. In the mix Jack comes across as a rather pallid presence, trying without much success to persuade everyone to get along. And late in the action there’s a revelation about why Emilia is taking the loss of her daughter so very hard, which comes across as rather calculated though the way it’s resolved leads to welcome layers in some of the characters.

The secondary figures, moreover, have a bookish feel that keeps them from being fully convincing. There are Emilia’s co-workers Mindy (Lauren Ambrose), whose own wedding bells cause brief turmoil, and Simon (Anthony Rapp), the almost obligatory sharp-tongued gay friend. And her long-separated parents (Debra Monk and Michael Cristopher), whose reconnection angers Emilia because she’s still furious about her father’s infidelity.

As might be clear from this, the parallels the script draws among the various characters can be unsubtle, and William frankly emerges as more literary conceit than authentic kid—the sort of tyke who’s always saying things that suggest he’s older than his years. But throughout Portman is excellent, going to great lengths to avoid easily making her character more sympathetic. And through Kudrow’s brittle nastiness can initially seem almost humorously overstated, in the end she gives the scorned wife considerable power.

“The Other Woman” can seem a bit oblivious to reality—one hopes, for example, that the ostracism of Emilia as a home-wrecker by the other mothers at William’s school is intended satirically (how many marriages in New York City last, after all?). Even the title is too head-on. But it builds cumulative strength, largely through Portman’s performance, and Roos and cinematographer Steve Yedlin use the urban locations to create a strong Big Apple atmosphere. The result may be essentially a soap opera, but it’s an entertainingly quirky one.