The substories will start populating the map when you go back to the apartment and it turns to daytime. They can also be done whenever. Nothing is permanently missable.
This is probably the best rhythm game I've played in years. It's amazing.
Beating the last level and boss felt like a real achievement. Going for the S ranks now which is pretty fucking intimidating, seeing as there's pretty much no one on the leaderboards with S ranks on the last few levels.
Larsa's comments