Anne Hathaway reliably enchants throughout her new movie, “The Idea of You,” having its premiere this week on Amazon Prime Video, but there’s one scene in particular in which she gives an acting masterclass in how to deliver a long monologue. 

The scene involves Ms. Hathaway’s Solène, a middle-age art dealer, recounting to Nicholas Galitzine’s Hayes, a 24-year-old member of a boy band and a budding suitor, how she was betrayed in her marriage. As she explains how she and her ex-husband met, the development of their relationship, the inevitable decline in affections, and how she came to find out about his affair, Ms. Hathaway draws on pauses, voice modulation, downward glances, and more to convey the character’s complicated emotional truth as smoothly and sincerely as possible. 

If only the rest of the movie were as brilliant and insightful as this scene. Billed as a “rom-com-dram” by director Michael Showalter, “The Idea of You” conjures up skin-deep romance and barely any comedy as it portrays its tepid drama. Its story of a love affair between an older woman and a young man may evoke other May-December movies like “The Graduate” and “Something’s Gotta Give,” yet the picture musters limited wisdom and charm. 

The little tension it does generate involves outrage culture and social media hysteria, yet even these are lightly dramatized. What we’re left with is a glamorous fantasy based on a book by actress Robinne Lee ostensibly inspired by Harry Styles, formerly of the boy band One Direction.

Solène and Hayes meet-cute at Coachella. Solène is attending the music festival as a chaperone to her daughter Izzy and the teen’s two friends. While there she enters Hayes’s celebrity trailer, thinking it’s a bathroom for VIP ticket-holders. The scene is mildly diverting, especially since both Ms. Hathaway and Mr. Galitzine have an otherworldly beauty, but the humor is indistinct, as it is in the next scene, which portrays the official meet-and-greet event for Hayes’s band, August Moon. Later, during the band’s concert, Hayes mentions to the audience that he met someone special and proceeds to sing a song while eyeing Solène, yet even here the filmmakers downplay the awkwardness she feels in a missed comic moment.  

At this point, one can probably guess the tack of this creaky story but here’s a plot summary: Hayes visits Solène’s Los Angeles gallery, purchases all the art in it, and then the two have lunch at her home, which is where the scene of Solène baring her soul takes place. From here, there are trips to New York and Europe as their relationship grows, until the inevitable interaction with band “groupies” leads to disillusionment and a breakup. There’s a reconciliation back in L.A., but then the press catches wind of the romance and the movie lists into melodrama and glib relevance with trending topics.

The portrayal of the physical relationship between the couple is one aspect in which the filmmakers succeed. The two stars definitely have chemistry, and there are multiple tastefully sensual, mildly erotic scenes. As evidenced in the recent tennis-themed love triangle entertainment “Challengers” and the Oscar-winning “Poor Things,” among others, Hollywood has re-embraced sex on screen after a downturn in prior years. 

Mr. Showalter, his co-writer Jennifer Westfeldt, and editor Peter Teschner make the mistake, though, of confusing scenes of intimacy and a couple conversations about prior heartache for romance. There are no scenes whereupon Solène introduces Hayes to her best friends, thereby grounding their relationship and expanding it beyond the physical. No dialogue exists in which they longingly discuss how they envision their future together, nor, for that matter, moments that playfully map out their individual idiosyncrasies and each other’s reactions. 

What we do get are many montages, like the one in which they cavort, scantily clad, to Wang Chung’s ’80s hit “Dance Hall Days,” a fun but not exactly telling sequence.

The biggest problem with “The Idea of You,” though, isn’t the lack of real romantic moments, but it’s near total dearth of jokes. There’s a revealing, self-defeating instance by Mr. Showalter when he has Solène watch “The Graham Norton Show” on television and a clip of the singer Adele is shown, the one during which she tells the talk show host that while living in the U.S. she misses British humor.

If only the film had what Adele hints at – a wittily irreverent, English take on the twosome’s relationship. Despite Mr. Galitzine being British in real life and in the movie, he’s given very few funny lines to work with, and the actor doesn’t seem to improvise. With its comic clash of cultures – American and British, modest living and celebrity extravagance – the movie could have been a fresh take on 1999’s “Notting Hill,” albeit with a cougar-ish twist.

Instead, we have a narrative that sinks its romantic and comedic notions in “lifestyle” touches and in its bid to say something serious about society’s view of middle-aged women. Yet it could have landed its theme more skillfully had it not relinquished its rom-com side. Which prompts the question: Where’s Hugh Grant when you need him?