"Kōtetsu no Hana" is set in a fictionalized version of Japan during the Meiji Restoration, blending historical elements with supernatural and steampunk aesthetics. The story centers on Hana, a young woman who becomes the host of a powerful, sentient armor known as "Kōtetsu." This armor, imbued with ancient technology and mystical properties, grants her extraordinary abilities but also binds her to a dangerous fate. Hana's journey begins when her village is attacked by rogue samurai and mechanical monstrosities, forcing her to don the armor to protect her loved ones. As she masters its powers, she discovers that the armor is tied to a larger conspiracy involving the remnants of the Tokugawa shogunate and a secretive organization seeking to control the nation's future.

The narrative explores themes of identity, sacrifice, and the clash between tradition and modernity. Hana's struggle to reconcile her humanity with the armor's growing influence forms the emotional core of the story. Alongside her are key allies, including Ren, a former samurai turned revolutionary, and Ayame, a skilled engineer who provides technological support. Antagonists include the enigmatic leader of the shadowy organization, who seeks to harness the armor's power for their own ambitions, and a rival armor-wielder whose tragic past intertwines with Hana's destiny. The plot unfolds through a series of battles, political intrigue, and personal revelations, culminating in a confrontation that will determine the fate of Japan and the legacy of the Kōtetsu armor.