Inspired by Chris Sievey's persona of Frank Sidebottom and the time the co-screenwriter Jon Ronson spend in Sievey's band, the film is about far more than the eponymous character wearing a big head, in the same way that any (good) band is more than the sum of its parts. Frank shows the complexity of the chicken-and-egg question about the origin of creativity. And then it complicates the question further by throwing in the the added dimension of collective artistic expression; this is about a band, after all. Think of it as a po-mo version of The Commitments where you spend less time cheering for band, and more time going back and forth between "WTF?" and "Huh, that's kinda deep."
I don't think it's spoilery to say the band breaks up. C'mon, it's a band movie--when was the last time a movie band didn't implode? But Frank might surprise you a bit with the whys and hows of the breakup, and might also surprise you with how the breakup leaves you feeling.