Ami Hyūga was an average high school girl, trying to lead a normal life but her world came crashing down when her brother Yu and his friend Takeshi Sugihara were killed by bullies led by Sho Kimura. As Ami tracks down Sho, she is surprised to discover the bullies' association with a sinister ninja yakuza family. When she goes after her revenge, she soon finds herself in over her head with her left arm lost as a result. Barely surviving, Ami escapes and seeks out shelter from Takeshi's parents Suguru and Miki Sugihara, two kindly garage mechanics who fit her with a high-powered machine gun prosthetic. With Miki's use of chainsaw, Ami proceeds on a killing spree against the murderers of their loved ones.