When he was a boy, Rosso's family was massacred by a vampire who turned him into a werewolf. In revenge, he became an investigator in the sixth division of the special police unit, assigned to cases involving vampires and werewolves. At the request of his superiors, he was forced to train a new investigator, Mikumo. Initially annoyed by Mikumo, Rosso and his new partner find themselves in an awkward situation. Having refused to drink non-synthetic blood, he is forced to drink Mikumo's to regain the full extent of his powers. But this also introduces a new, unknown feeling.