"Manyū Hiken-chō" is set in an alternate historical Japan where the size of a woman's breasts determines her social status, power, and influence. The story revolves around the Manyū Clan, a powerful organization that controls the distribution of a mystical substance known as "Chichi," which enhances or diminishes breast size. The clan enforces strict hierarchies based on this attribute, with larger-breasted women holding dominance over those with smaller breasts.

The protagonist, Chifusa Manyū, is the heir to the Manyū Clan. However, she rejects the clan's oppressive ideology and seeks to dismantle its system of control. Chifusa wields the titular "Manyū Hiken," a secret sword technique passed down through her family, which allows her to manipulate breast size. Accompanied by her loyal attendant, Kagefusa, Chifusa embarks on a journey to challenge the clan's authority and liberate women from its tyranny. Along the way, she encounters various adversaries, including members of the Manyū Clan who seek to maintain the status quo, as well as other factions with their own agendas.

The narrative explores themes of power, societal expectations, and rebellion against oppressive systems. Chifusa's journey is marked by battles, both physical and ideological, as she confronts the deeply ingrained beliefs of her world. The series blends action, humor, and social commentary, presenting a satirical take on societal beauty standards and the objectification of women. Through its exaggerated premise, "Manyū Hiken-chō" critiques the ways in which physical attributes are often tied to worth and power in real-world societies.