“Prime” is about a woman who starts dating her psychiatrist’s son, and none of the three people involved realize it. How that can happen is a marvel of screenwriting: The writer has to be very careful about what these three people say to each other, lest everything be revealed sooner than he wants it to be. The son can’t mention to his mom what his new girlfriend’s name is, and the woman can’t tell her psychiatrist the full name of the man she’s dating. Oh, and the psychiatrist and her son need to have different last names.

I think Ben Younger, who wrote and directed the film, has all his bases covered. Which isn’t to say that the whole scenario is plausible, merely that it’s possible. Even when the truth does come out, there is still plenty of time left to deal with the aftermath. Younger keeps the focus on the characters as people, not just as pawns in a silly movie plot. I like that a movie with such a farcical premise can be so upscale and witty.

Uma Thurman plays the woman, Rafi, 37 years old and recently divorced. She meets David (Bryan Greenberg) through a mutual friend and begins dating him, recognizing that he’s younger but not knowing how much younger until she finally asks: He’s 23. Yikes. They both think the age difference is substantial, but they both know they like each other, too.

Meryl Streep plays Rafi’s psychiatrist, Lisa, a very Jewish woman who, like all New York psychiatrists (at least the ones in movies), wears big goofy necklaces, out-of-date eyeglasses and a frumpy hairstyle. She’s delighted that Rafi’s dating someone, and since Rafi tells her that the man is 27 — she’s too embarrassed to mention his real age — Lisa doesn’t realize that Rafi’s David is HER David.

Meanwhile, Lisa is berating David for dating a girl who, by his description, is not Jewish. Mom is insistent that there’s no point in dating someone you have no intention of marrying, and if he plans to marry a shiksa, he’ll do it over her dead body. David doesn’t argue the point; he’s just dating for fun right now. He’s young and carefree, so why not?

So we know more about the characters than they do, and there is much humor in seeing them discover it, one at a time, slowly but surely. Lisa is pleased to hear Rafi talk about her sexual exploits with David; you can imagine how that changes once she figures out who David is. Even simple things like Rafi’s discovery that her too-young boyfriend lives with his grandparents (he had to get out from under his mom’s thumb) can be hilarious.

You gotta love Meryl Streep, and you gotta love Meryl Streep doing comedy (see “Adaptation”), proving that a great dramatic actress can be funny, too, by employing the same rules: Be real and be honest. She’s paired here with Uma Thurman, who’s also not famous for being funny, but who is perfectly capable of it. I like the dynamic between Lisa and Rafi more than the one between Rafi and David, actually, and it’s reassuring to see that the movie winds up placing as much emphasis on the two women as it does on the romance. (Bryan Greenberg, the likable fellow who plays David, is understandably overshadowed by Streep and Thurman.)

The romance runs into snags because of the age difference: David is youthful and irresponsible, while Rafi has already passed that phase of her life. There is the suggestion that maybe love doesn’t always conquer all — but then again, maybe it does. Or maybe it doesn’t. Maybe this movie has too many scenes where Rafi and David break up, reunite, and break up again. Maybe it should give it a rest.

It’s enough of a romantic-comedy to satisfy fans of that genre, but different enough from the usual template to be worth recommending to regular people, too. It’s also a good reminder not to lie to your shrink.

B (1 hr., 45 min.; PG-13, a little profanity, one F-word, a bit of sexuality and some sexual dialogue.)