Sometimes humans do something so terrible, so horrible, that the world cannot move on. The heaven's weep and send Tsukidai's kind down to a world, to wipe the memories of those affected, and turn the memories into butterflies.
This is why people many feel emotional when they butterflies. In time fluttering about the world, the memories are purified and absorbed into the earth.
Tsukidai searches for the sorrow that created him, eternal until he cures it, and then disappears like a shadow in full moonlight.
There are two kinds of people in this world, those that work to ease the pain of others, and those who cause more pain to the world. If you do nothing to help, you're only causing pain.
The sorrow of the world is growing...