To Meryl Streep's fans, she is the most assured actor of her generation. Who could better convey the anguish of a mother forced to choose which child should die in "Sophie's Choice"? What other performer could make it even plausible that a woman would desert her son in "Kramer vs. Kramer"? Would "Angels in America" have taken flight without her in several masterful roles? Then there's the other Streep faction that considers that mannered style and those multiple accents little more than dramatic showboating.

Surely even her detractors will acknowledge Streep's richly comic significance in "Prime." Technically, Ben Younger's romance is about two mismatched lovers, played by Uma Thurman and Bryan Greenberg, who try to give love a whirl despite their age and religious differences. But as the therapist all too closely involved in this affair, Streep's the giant bay leaf in this stew.

Rafi (Thurman) is 37 and recently, messily divorced. She's looking for a new partner and can hear that baby clock ticking. When she meets 23-year-old David (Greenberg), who isn't even wise beyond his years, she's still attracted to him. He's certainly not father material, but he offers her something she hasn't enjoyed lately: unconditional reverence. A girl can have fun, can't she? The trouble is, Rafi finds herself falling deeper for someone whose idea of a good time is playing Nintendo.

Luckily, she has Lisa (Streep), a devoted therapist who listens to Rafi's deepest, juiciest confessions with nonjudgmental serenity. Lisa's a little wary of the mismatch -- despite Rafi's adding a few years to David's age -- but she's glad her client's finding happiness so fast.

Ah, but the piquancy of this movie is neither the age gap nor Rafi's lying. It's the sticky connection between the women: Neither realizes that Rafi's lover is Lisa's son. Lisa, devoutly Jewish, believes no one should stray outside his religion. Nor would she want David sleeping with a goy like Rafi or anyone she hadn't personally vetted for, well, everything. David hasn't made the connection either, but he's aware that Mom wouldn't be thrilled to hear about him dating a thirtysomething shiksa who owns T-shirts older than he is.

When Lisa puts it all altogether -- way before Rafi -- she continues counseling her (at the advice of her own therapist) as if nothing is untoward. She cares about Rafi and wants her to be happy; and she figures she can probably gently steer her client out of this.

"Prime" doesn't have the mastery of such breakout romantic comedies as "Annie Hall," "Broadcast News" and "Moonstruck." But it shares their desire to deepen the genre with larger issues. And by writing about different generations, Younger (who also made the 2000 "Boiler Room") cracks open the genre. You're not just watching three characters in a farce, you're witnessing the whole, excruciating trajectory of life. You're acutely aware of David's immaturity and his noble aspirations. You can feel the real panic in Rafi, as she weighs the pros and cons of a life with a younger man. Streep's vehement opposition is not the reflexive resistance of a comic heavy, it's the sincere reaction of someone with a strong belief system. There's an underlying seriousness here that makes the comedy richer and sets things up for the Meryl show.

As Lisa, she's the movie's most entertaining nucleus -- love her or hate her. While listening to Rafi's increasingly lurid tales, she writhes in her seat as if she's about to be strapped to the electric chair. She paces the office pretending to search for a book to hide her flustered state. She chugs glasses of water like a drowning victim. Her eyes seem to spin in different, despairing directions. And she does all this with hardly a word. Thurman and Greenberg may be the ones doing all the giggling, but Streep's the one making us laugh. Comedy, someone once said, is about reactions not actions, in which case, the grande dame's still very much at the top of her game. Maybe even her naysayers, should they bring themselves to watch this movie, might begrudgingly raise a thumb.

Prime (105 minutes, at area theaters) is rated PG-13 for sexual scenes and profanity.

Bryan Greenberg and Uma Thurman as the doctor's son and his older girlfriend in the farce "Prime."

Meryl Streep plays a therapist whose son is having an affair with her patient.