John Carney has made a career of basically finding a frame story to give him reasons to string would-be music videos together. I'm not saying this is a bad thing. He's good at it, and I've liked two of the three movies I've seen. It's just very noticeable. By the time I got to Sing Street, as cute as the plot is, I had come to realize it was more about the music than the characters.
Feel free to argue. The story of Sing Street is that in 1980s Dublin, 15-year-old Conor wants to impress a girl he sees sitting on a stoop. She tells him she's a model, and he randomly asks if she wants to star in his band's video. Which means he needs to start a band and shoot a video. Then comes the crash course in learning to write songs, etc., and a series of music video shoots.
Sure, there's a bit more to it. The would-be romance between Conor and Raphina, the stresses of a family falling apart, the school bully—or bullies, if you count the administration as one. But none of it feels all that important. They're there because there needs to be mortar to fill the gaps between the bricks that are the musical numbers. Mortar is important to a wall, yes, but the bricks make the wall. You dig?
Again, it's not necessarily a bad thing. I actually really enjoyed the movie, and it's a toss up between this one and Begin Again for me. I like them about equally. (I tried to watch Once, though, and couldn't get through it. Sorry. Know it was an award-winning whatever but not for me.) Bottom line is, John Carney movies are this one thing. When you're in the mood for that one thing, you watch one. Or maybe you skip the movie and just buy the soundtrack.