The aging but still potent charm of Hugh Grant is the saving grace of this flyweight but mostly agreeable romantic comedy, which is also buoyed by its jovial but good-natured send-up of pop music past and present. “Music and Lyrics” is no Mozartean masterpiece, but it’s a cheerful little ditty of the sort that you enjoy hearing on the radio but won’t remember afterward—the cinematic equivalent of bubble-gum music, splashily shot (by Xavier Perez Grobet) and nicely mounted but more like a plastic flower than a real one.

In Marc Lawrence’s jovially predictable screenplay, Grant plays Alex Fletcher, a has-been singer whose claim to ’80s fame rests on his high-profile membership in a once-popular quintet named PoP (modeled, it seems, after Wham!) that broke up and left him adrift. He’s not angst-ridden over his fate, though; he’s remarkably upbeat doing the nostalgia gigs arranged by his devoted manager Chris (Brad Garrett), though even those are proving harder and harder to land. (Indeed, his only prospect at the moment is an appearance on a gruesome CW reality series called “Battle of the ’80s Has-Beens,” which turns out to be a boxing program.) That’s why it’s almost miraculous when teen superstar Cora Corman (Haley Bennett) asks Alex to write a new song for her titled “Way Back Into Love,” which he’d perform with her at an upcoming concert.

There’s a problem, though: he has no lyricist. But one shows up providentially in the form of Sophie Fisher (Drew Barrymore), a ditsy replacement plant-waterer who just happens to show a natural gift for the trade, having studied poetry in college. Unfortunately, she’s beset by personal problems—a sad affair with a professor (Campbell Scott) who unchivalrously used her as the model for the clingy heroine of his prize-winning novel—and has to be persuaded to serve as his collaborator. When they finally do link up professionally, of course, they’re attracted to each other as well, but after Cora threatens to add her own imprint to their song, their disagreement over how to react puts a roadblock in the way to romantic fulfillment. It doesn’t take a genius, musical or otherwise, to foresee that an extravagant finale at the big concert will reconcile them.

It’s a pretty flimsy conceit Lawrence has fashioned here, and he doesn’t direct it with much panache, especially in the second half, which does tend to run down like a crank-operated photograph that’s losing steam. But he’s fortunate in having a cast that, by and large, can make the material seem better than it really is. Grant’s graying boyishness is very likable here, and his rapid-fire delivery makes his sardonic lines work as the throwaway zingers they are. Garrett’s hangdog persona complements him nicely, and the Amazonian Kristen Johnston (from “3rd Rock from the Sun”) does a wonderfully over-the-top turn as Sophie’s voluble sister, a big PoP fan and matchmaker, too. Unhappily, Barrymore doesn’t match Grant in the movie’s romantic equation. It’s not entirely her fault, since Sophie is a poorly-written character whose mixed chatterbox/doormat personality gives her little to work with. Scott is a cipher in what amounts to a cameo, and though Bennett is striking in her performance scenes, she’s less so in the conversational, where her almost robotic delivery is certainly an intentional parody of some present-day pop divas but is still pretty drab.

Where the strength of “Music and Lyrics” lies is demonstrated, quite appropriately, in the song interludes. The best comes at the very start, when the credits run over a marvelous faux music video of PoP’s signature tune, “Pop Goes My Heart,” which ridicules the music of the period, but does so affectionately. None of the rest equal it, not least the songs featured in the concluding concert, though they’re good enough. The movie follows the same trajectory. It starts off with a bang but trails off as it runs on, winding up overall a mildly amusing trifle.