I killed a girl. Who was she...?

The protagonist, Kunimi Kyousuke can't remember the memories of his distant past. Even the details are vague, merely appearing as flashes in his heart.

"I am certainly the one who with this hand did that to the lone girl", he keeps thinking, but can't even remember the time or place it happened. But the memories are slowly entering his mind. He remembers the corpse...

How will Kyousuke deal with his supposed sin?