Review: ‘Power Ballad’ is a one-scene wonder with a terrific start and a sloppy second act

“Power Ballad” is a cynical, sloppy comedy about a wedding singer (Paul Rudd) and a pop titan (Nick Jonas) who drunkenly tinker with an unfinished song, then squabble over who gets custody when it becomes a hit.

Disharmony is a new chord for filmmaker John Carney, who has specialized in films about collaboration since his 2007 art house hit “Once.” From the snotty ’80s synth music of “Sing Street” to the acoustic heartbreak of “Begin Again,” Carney loves to hear how a rough idea evolves into a polished track. Lately, he’s been trying to find new approaches to his formula. His 2023 “Flora and Son” was about a surly single mother who picks up a guitar and discovers that her life doesn’t change that much. I loved that one because it said we’re all entitled to noodle without having to make much fuss about it.

The problem with “Power Ballad” is that it’s all rough ideas itself. Like an album that kicks off with its single, it starts with a jazzy sequence of song creation that’s one of the best versions of the scene Carney has ever done. A faded teen icon, Danny (Jonas, gamely sending up his history as one of the platinum-selling Jonas Brothers), explains how to write a PG-rated hit to Paul Rudd’s cover band frontman Rick, who immediately tries to improvise lyrics with the word “titties.” (Or as he warbles it, “tit-taaays!”). It’s perfect casting — Rudd’s singing voice keeps up with his comic chops.

Fifteen years ago, Rudd’s Rick fancied himself an up-and-coming American rock god until his grunge act went on a transatlantic tour and he fell in love with an Irish girl, Rachel (Marcella Plunkett), and had a daughter named Aja, a feisty teen played by the scene-stealing Beth Fallon. Settling into a life of anonymity in the Dublin suburbs, Rick now has his wedding band Bride & Groove, which allows him brief flashes of cool — or at least the chance to strut into gigs wearing tight black jeans. His bandmates are pretty funny but so underwritten that one even gives a big speech about how he’s more than just a bit player in Rick’s life.

Straight away we’re suspicious that Carney and co-screenwriter Peter McDonald stashed this story in a drawer ages ago and didn’t bother to dust it off. If Rick quit his serious group during the Obama era, why does he consider himself a peer of Eddie Vedder? The character is written to be in his late 30s, but acts decades older than that, posturing like he’s one of the last ambassadors of authentic rock and roll while ripping through a setlist that’s mostly Hall & Oates. (Shouldn’t he at least be jamming the Killers’ indie-sleaze wedding staple “Mr. Brightside” that sends bridesmaids shrieking onto the dance floor like a maenad cult?)

At one soiree, Rick is asked to pass the mic to Danny, a boy-band celebrity struggling to make the leap to a solo artist. Rick has already mocked him earlier that day as “the death of the music industry,” but Danny proves to be a true performer with enough star power to electrify Rick, who starts copying his gestures, clapping alongside him and having a blast. Their chemistry carries over into an all-night jam session in which Danny and Rick share spliffs, whiskey and scraps of songs that they haven’t managed to perfect.

For a thrilling moment, the movie is a platonic rom-com about two dissatisfied artists coming together from opposite directions: Danny crumpling under mass scrutiny, Rick weary of obscurity. Danny is desperate to get back to playing Madison Square Garden; Rick has long since abandoned his dream of playing there even once. Having seen a movie or two before in my life, I assumed that “Power Ballad” would climax at that stadium with Danny and Rick harmonizing for a crowd of 15,000. Corny, sure, but satisfying.

But this banger of a bonding session is a one-night stand. In a shallow heel turn, Danny poaches one of Rick’s unfinished songs and blows it up into a chart-topper. It’s a major betrayal for Rick and a bummer for the audience, who never get to see the two make music together again. Instead, Jonas’ Danny becomes a callow Hollywood creep just like his manager, a menacing slickster named Mac Darling (Jack Reynor), who seems hip until he tries to explain an internet meme and it becomes obvious that Carney doesn’t understand what a meme is.

Meanwhile, Rick suffers a meltdown, haunted by the hit he can’t escape. The stolen track chases him everywhere: on the radio, overheard at the mall, even at his own gigs where newlyweds cluelessly ask him to play “their” special song. We’re forced to hear endless snippets of it, too, although the full lyrics are saved for the end when we discover that one of songwriters clumsily shoved in the word “albatross.”

“Power Ballad” nods toward a dozen interesting themes, none of which it bothers to explore. It could be about what turns a pretty melody into a mega-smash, about the value of songwriting versus charisma, about timid artists who hoard their best material and showboats obligated to satisfy their promotional teams, or even about how a song ultimately belongs less to its creator than its fans. It also flirts with being about how both men write for female approval — girls are Danny’s fanbase and Rick’s family is his entire world — only to have their women think most of their songs are pandering and dull.

“What are you interested in?” Rick finally huffs to his daughter.

“Revenge,” she says.

Well, Carney’s made a movie about darkness and it’s a total bummer. In a brutal little detail, he contrasts Danny’s bikini groupies with the women in Rick’s crowd, who clomp toward him with toilet paper stuck to their shoes.

“Power Ballad” postures like a sincere drama but has the set pieces of a giant slapstick farce. Rudd seems to have been told he’s in one, playing Rick’s humiliation so large that he looks unhinged, his face covered in cuts and bruises of his own doing that, distractingly, never seem to heal.

The result is punishing — and tone deaf.