The setting is modern Japan. Mikage Azusa is a ninja who belongs to "Oboro", a ninja group that's been protecting people's peace from the shadows. For many years, she's been fighting against "Meigas", a giant criminal organization. After being repeatedly defeated by Oboro's ace Azusa, Meigas starts a plan to develop the ultimate biological weapon which receives the name of "O-Plan". Thus is how the Orc, a biological weapon that possesses physical and regenerative abilities that far exceed those of any human as well as a monstrous lust, is born. Azusa loses in battle against the orc and becomes Meigas' prisoner. Ending up as a breeding partner for orc breeding experiments and against an orc's nearly infinite stamina and lust, Azusa eventually starts to awaken to the pleasures of being a "female"...