Zwei lived as an average individual until a fateful night when he witnessed a savage murder while strolling through the streets. Although he managed to evade being shot by the shooter, a girl called Ein, he was ultimately abducted and compelled to undergo an episode of memory loss. His life was spared with the proviso that he learns the craft of assassination under Ein's tutelage. Presently, he is determined to disengage himself from the murky domain and restore his existence to its former state.