It's on the end of chapter 4. All you have to do is to continue with the story. I tell you, that's the best video song ever made! I love it!

I can't say more or I'll spoil the fun!

Important note: you have to watch all the Coom spheres to get some percentage points, it's real useful! But it's also annoying... but have some patience on that.

And try to level up also, because soon you'll have some hard battles!