Planck, Shadegrown Games' music-based shooter, but this video should go a long way towards explaining exactly how it works. As you see after the break, the idea is that you're traveling along shooting items that release "trapped" sounds, building a music track as you move along. There's also a sort of boss battle at the end, though this interview with Shadegrown's Matthew Burns hints that it's a "special encounter," designed to play with the tone of the music rather than the notes themselves.
Sounds interesting. Shadegrown hasn't mentioned a release date target or even platforms for the title yet, but we'll keep our ears open and let you know when we hear something about it. Because it's a music game. Which you listen to. Get it?
Update: Shadegrown has given us both a platform and a date for release: The game will be out for PC (with other platforms to come) and probably in 2012.