The leader of the Kouga ninja believes that the Iga ninja will try to undermind the government (costing the Kouga their poisition of power) so he order his kunoichi (female nija) Okou to get close to Hattori, Hanzou III. She gets a job in Hanzou's bathhouse. Hattori, Hanzou II killed Okou's father so she's expected to eventually take revenge by killing Hattori, Hanzou III.
Someone starts killing Kouga ninja. Their leader assumes its Iga led by Hattori, Hanzou III. Hanzou learns its a childhood friend who thinks the Iga should regain power by force. Hanzou believes such actions will bring down the wrath of the government and the deaths of all the Iga.