Time for a case study. The patient is a 35-year-old actress coming off a difficult divorce. After spending a few years slaughtering her enemies in an ironic pop-fusion action flick, she’s looking to try something new. What to do?

For Uma Thurman, the answer is: take a role as a 37-year-old woman in therapy coming off a difficult divorce. And in director Ben Younger’s flat romantic comedy “Prime,” Rafi (Ms. Thurman) tries something new, all right: shtupping a 23-year-old hunk of “prime” beefcake.

To spice up this dinner-table anecdote of a premise, the kid is the son of her psychiatrist (Meryl Streep). Mom catches on after a few sessions and doesn’t cotton to her nice Jewish son, David (Bryan Greenberg), dating outside the faith. Middling culture-clash comedy ensues, playing off both differences in age and background.

“Prime” is a little like David: plain, sweet, and supposedly talented. Seeing Ms. Streep and Ms.Thurman work together is a pleasure, and the oh-my-god premise and classical plotting provide some good fun. But what early on looks like confidently restrained direction and, for Mr. Greenberg, a pleasing plainspoken manner, eventually develops into nothing special.

Rafi’s worst fears might be right: There’s just nothing more there.

For one thing, it’s a huge problem for a romantic comedy to have so little chemistry between its stars. Not Ms. Streep and Ms. Thurman – they get on smashingly. The main problem is Mr. Greenberg, who can’t hold a candle to his luminous co-star. He’s superficially convincing as a sensitive young man, but essentially procrastinates his dullard charm: He turns on the attentiveness only when the scene absolutely commands it.

Rafi and David’s hungry courtship does throw off some nice warmth, though, even if one passionate kiss is ickily paired with some romantic quail-egg slurping at a Japanese restaurant. But as David loses his housing and moves in with Rafi – following the inexorable New York calculus of relationships and real estate – it’s too much for the story to bear. And throughout, for all we learn of David, Rafi remains a whopping cipher.

Which in a sense makes Ms. Thurman’s performance all the more gripping and immediate – especially in the film’s most successful scenes, the therapy sequences between Rafi and her Streep-shrink. In terms of storytelling and emotion, these prove a remarkable dramatic back door to Rafi’s character. In most romantic comedies the hero or heroine has a friend, and their chats reveal their hopes and fears. Rafi has one of those, too, but the really juicy bits come out in her therapy sessions, and Ms.Thurman goes all out. As Rafi confesses every last one of her feelings and experiences – from her love of “sweet” David to wanting to knit a hat for a certain body part – it’s a wonderful portrait of soul-baring, charged with the trust and vulnerability of therapy. It’s like going behind the scenes of the romantic comedy.

Of course, these sessions are also intended to provide the finest comedy of the movie. And, though predictable, the farce of Mom hearing about her son’s sex life is good for a guffaw. Ms. Streep again demonstrates how to make even a thin screenplay work, showing good comic timing with details like repeatedly reaching for water to retain composure.

Mr. Younger doesn’t come close to making a simple version of the “Annie Hall” he remembers in the back of his mind, though he cribs the ending montage set to a wistful jazz vocal. Rothko references and a subplot about David’s art career ring tellingly false, and the stereotypical supporting characters (such as, disastrously, Rafi’s gay friends) are painful, not affectionate. But worst of all, he flubs New York: Mom spies on her son and Rafi at … Crate and Barrel. Even if that is Gotham today, I don’t want to know about it.