Humanity's only form of defense against the D2s are Musicarts, young women representing pieces of classical music and Conductors, the ones controlling them. A public decree banned citizens from playing any melodies, to prevent further casualties caused by the D2s' hatred for music-even now, in 2047, this prohibition is still in effect.

The United States of America has been in chaos ever since the emergence of D2s, an invasive species originating from a black meteorite that fell to Earth.