In spite of the least attractive title in ages, "Prime" is two-thirds of a marvelous movie. Those two are the performances of Meryl Streep and Uma Thurman, who between them almost carry the film; it's just that the third third, you might say, is the performance of a young actor named Bryan Greenberg, who has no charisma, slurs his line readings, and isn't even cute. It's a performance so pale it fades to near invisibility whenever he's on screen with the others.

Nevertheless, there are charming moments spotted throughout the film, which is the story of Rafi Gardet (Thurman), newly-divorced 37-year-old, in therapy with Lisa Metzger (Streep), who meets 23-year-old David Bloomberg (Greenberg) and falls for him, as he does for her (and who wouldn't?). Sex is great (Lisa points out that at their ages they are both in their sexual prime), but the question is whether it can last. And then, of course, it turns out that young David is in fact the son of Ms. Metzger, who is treated to Rafi's most explicit descriptions of their sex and must then go running to her own therapist for help.

Rafi naturally wants to meet his family, but David is mortified to let her know that he is living with his grandparents, and complications, as we say, ensue. In fact one of the great moments in recent film history comes as David sneaks Rafi into the apartment (he's told her that he has 'roommates') and he must call out "It's me, Grandma," at which point Thurman does a swan-like swoon, sinking gracefully to the floor as she realizes what she's gotten herself into. That moment alone will almost make the film for you.

Lisa does a great deal of worrying about the fact that Rafi is a shiksa and there won't be Jewish grandchildren if she and David marry. On the other hand, we know that this film will not have a Demi Moore-Ashton Kutcher resolution, so we can sit back to watch how it will play out. Writer-director Ben Younger (he made the remarkable first film "Boiler Room" in 2000) gives us lots of nice scenes and moments, but every one of them is flattened by the performance of Mr. Greenberg. And yet, to watch two great pros - Streep and Thurman - move so sweetly through their scenes together, without a moment's awkwardness or any attempt at grabbing the spotlight or upstaging the other, is one of the delicious pleasures of the film. They almost make it worthwhile.