"Prime" is a curious romance, maturely developed and kind of sweet on the one hand, but also unclear on where it is heading, what it hopes to accomplish, and why. Beginning as a low-key slice-of-life, moving into sharply comic and often uproarious territory for its second act, and then finishing off as a bittersweet love story, writer-director Ben Younger's (2000's "Boiler Room") screenplay is schizophrenic in tone and, ultimately, lacking in some much-needed clarity.

37-year-old Rafi Gardet (Uma Thurman), a steadfast patient of therapist Lisa Metzger (Meryl Streep), is thrown for a loop when she finds herself falling for 23-year-old struggling artist David Bloomberg (Bryan Greenberg) just a week after finalizing her divorce. The sizable age difference concerns Rafi, and David, meanwhile, is being pressured to find a Jewish girlfriend—something that Rafi isn't—but there is no denying that they click together. What Rafi doesn't yet know is that Lisa is David's mother. In hopes that their relationship is just a fling and caring about Rafi's well-being, Lisa opts not to expose her true identity, even as the in-the-dark Rafi begins sharing her and David's most intimate details during their sessions.

Once Lisa discovers the truth about the younger man in Rafi's life, "Prime" takes a turn toward wickedly sly, dialogue-laden comedy. There are several huge laughs as Lisa must contend between being an open-minded therapist and an outraged, concerned mother whose patient is suddenly the new woman in her son's life. Also, some choice flashbacks with David's senile grandmother are incendiary delights, especially her reaction to David bringing home an African-American girlfriend. The picture takes off during these fleeting sections, and seems to be headed toward a plot similar, but smarter, to 2005's "Monster-in-Law," with Lisa considering sabotaging the lovebirds' newfound happiness. This, however, never happens, and once Rafi experiences the revelation of Lisa's relation to her boyfriend, director Ben Younger loses a grip on how to handle the story he has taken an hour to set up. Lisa is mostly forgotten about, her place within the story becoming an afterthought as Younger opts toward a more conventional romance between an unlikely pair who may or may not be destined for a long life together.

As "Prime" loses focus in the last half-hour, throwing away the potentially provocative dynamic between Rafi, David, and Lisa, at least the May-September romance between Rafi and David is treated with a modicum of truthfulness. Questions involving how much age and religious differences play a part in a couple's relationship are broached in complex ways, and Uma Thurman (2004's "Kill Bill: Vol. 2") and Bryan Greenberg (2004's "The Perfect Score") share the requisite chemistry with each other to make what happens to them amiably involving. Only when a silly conflict arises in the form of a sexual encounter that occurs while the two are taking a break does the script dissolve into unnecessary cliches.

No issue can be taken with the uniformly fine main three performances, either, although how one of them is used is a wasted opportunity. The always-brilliant Meryl Streep (2004's "The Manchurian Candidate") is a comic firecracker as therapist Lisa, her reactions and line deliveries priceless as she eavesdrops on David and Rafi and tries to shield her added knowledge from Rafi during their candid conversations. Even as she gets to show off her funny side, even someone of Streep's high caliber finally cannot compete with the inconsequentiality of her character. When all is said and done, Lisa is virtually pointless to the rest of the story, and fades far into the background by the conclusion. To waste Streep's talents in such a thankless part should heretofore be a criminal offense. The film really belongs to Uma Thurman's Rafi and Bryan Greenberg's David, and they are both in top form. The more experienced Thurman, especially, is aging nicely into an actress of genuine warmth and depth, rather than simply playing the unobtainable object roles that she once did a few years ago.

"Prime" uses a welcome subtle hand in wrapping up the relationship between Rafi and David, but it does this at the expense of ever finding a central theme or purpose to make the film time well spent. What was the meaning of the Lisa character, for example, if nothing of interest was going to be done with her? To an even larger degree, and without giving away the ending, what was director Ben Younger's aim in telling the rest of the story? "Prime" is a motion picture of definite multiple personalities and a sinking waywardness. Very funny, but only in clumps, and sometimes emotionally effective, but in a minor way, the film has a difficult time in figuring out where its aim lies. By the underwhelming finale, it's still searching.