A girl born in Japan's Warring States period 400 years ago aimed to end the era as a ninja. Infiltrating a lord's mansion to assassinate him, she discovers he is actually the alien Mighty Black. As a mere human ninja, she stands no chance in battle, is left half-dead, and cruelly tormented in a pitiful state. However, she accidentally activates Mighty Black's transformation stick, turning herself into Cosmo Woman. Barely escaping, she flees to the present day, living as a schoolteacher. But Mighty Black sends agents to the modern era to retrieve the transformation stick. Cosmo Woman returns once more to the past to fight, yet despite her valiant efforts, she is captured and forced to return the stick. Handing it over would reignite the war, but refusing subjects her to degrading torment. Torn between the physical pleasure of her womanhood and the heroic duty of her mission, she battles with inner conflict. This clash of contradictions forms the core appeal, captivating viewers.