[Source: AP]

Let’s just say that the wedding band has never occupied the most exalted rung of the ladder in music.

Playing “September” and “Celebration” is often what’s most required. As one member of the Bride and the Groove, the band at the center of John Carney’s new film, puts it: They’re not rock stars. They’re human jukeboxes.

But in “Power Ballad,” a wedding band singer and pop star cross paths. For one night, all of the stratification of the music world falls away. “Power Ballad” starts like a fairy tale.

Since 2007’s “Once,” the Irish writer-director has focused his films on the redemptive capacity of music. Carney, who was once a bassist for the Frames, knows from experience. From “Sing Street” to “Flora and Son,” he has made unabashedly earnest tales where a song, or just picking up an instrument, changes lives.

This can, undoubtedly, lead Carney into sentimental territory. Lucky for him, his chosen subject — music — is more worthy of sentiment than almost anything else. Yet the song doesn’t quite remain the same in “Power Ballad,” a movie that begins with the gentle sweetness Carney is known for, but detours into something more discordant.

Rick (Paul Rudd) is an American musician who gave up on his once-promising rock band’s future to instead live with his wife (Marcella Plunkett) and teenage daughter (a spunky, underused Beth Fallon) in Dublin. His former group was called Octagon, a perfect former band name if there ever were one.

But for years, Rick has fronted the Bride and the Groove. It’s an unromantic day job (or rather a night one) that hasn’t entirely sapped his belief in his own songwriting. During an encore at one wedding, he plays an original tune and is mentally transported to an arena full of swaying fans. When he snaps out of it, he’s staring at an empty dance floor and faces that say: That wasn’t Kool & the Gang.

At another wedding at at a castle, the band is asked to let a friend of the newlyweds sit in. They reluctantly agree, and are surprised to see the very popular boy band veteran, Danny (Nick Jonas), step on stage. He sings Stevie Wonder’s “I Wish,” and it’s great. Though Rick had just dismissed Danny’s music as “manufactured content for young, excitable teens,” he discovers Danny is a genuine musician.

But, later that night, something even more remarkable transpires. Rick bumps into Danny, and the two quickly hit it off. They begin jamming together and sharing songs that need work. They are both so jazzed by their unlikely collaboration that they play into the next morning.

The actual moment of artistic creation, and the craft it requires, is something the movies almost always skip over. But capturing collaborative juices flowing is exactly what Carney excels at. You can feel his joy in it. So it’s fitting that one of the unfinished songs Rick plays for Danny, “How to Write a Song (Without You),” is about creative invention.

It’s here when you wonder where “Power Ballad” is headed. Is this, for Rick, the beginning of a beautiful friendship? Will they turn into the next great songwriting duo, lifting Rick out of weddings and proving to the world that Danny is more than a boy-band pretty face?