Imagine a future where the secret to immortality has been discovered, causing women to stop aging by the time they turn twelve. Not quite right, is it? That's why the Big Man Upstairs sends Dokuro-chan, an angelic assassin, into the past to "deal" with the sicko who messed with nature.
But she decides instead to move in with her target to keep him too occupied to find eternal life. The path of nonviolence leads to a fountain of blood, as she constantly kills him by accident, only to resurrect him with a magical chant moments later.