After her parents were brutally murdered, school girl Sawa was taken into custody by Akai and Kanie, a pair of detectives assigned to her case. Corrupt and immoral, they train the girl to become a weapon, dangling the promise of vengeance in front of the hapless orphan. From celebrities and politicians to influential businessmen, Sawa is tasked with assassinating targets selected arbitrarily by her crooked overseers. She executes every mission without fail, and her distinctive weapon has become infamous among the city's police officers. Physically abused by Akai, who is no more righteous than her victims, Sawa begins to dream of a life unhindered by the shadow of her "guardians." One day, Sawa meets Oburi, a fellow orphan and vigilante. They quickly form a bond born of desperation and disgruntlement at the unjust world, envisioning a future free from the stain of murder.

When her family is overthrown in a coup d'état, Princess Mayuka is sent to Earth to recharge the Ultimate Weapon. After studying the mating behaviors of Earthlings, she decides to enlist the help of one of her research subjects, college student Yoshizumi, but crash-lands on his roof. Taken hostage and shot with her own gun by Yoshizumi, she is brainwashed into thinking she is his younger sister. Fortunately, Yoshizumi grows fond of Mayuka and decides to help her with her mission: to awaken the powers of Akiho--the red-haired, green-eyed, appetite-driven Ultimate Weapon--using "primitive stimulation". (Source: ANN)

For young men like Kei, life is still plagued by nothing but homework and girl trouble. When an unknown force besieges Earth with robotic war machines, however, it is Kei who is chosen to defend the human race.