Lynn Suzuki, nicknamed Kotetsu (translating to "Little Iron" or "Little Tetsu"), hails from Deep Kyoto. After enduring harsh servitude under a cruel mistress, she flees to Tokyo (Edo) on a mission to locate her long-lost brother Tetsujin, last traced to the Kuon Detective Agency.

Kotetsu wields mastery over "kirohatchi," a lightning-fast quick-draw sword technique decisive in battles, including a clash against a tree-nymph antagonist. Her speech reflects her rural Kyoto roots through exaggeratedly formal and ornate language, rendered in adaptations like the English dub as a Cockney accent.

While investigating a mysterious device or dice on Tokyo’s streets, thugs confront her until detective Miho Kuon intervenes. Miho offers sanctuary and employment at her agency, dubbing her "Kotetsu" as a nod to her brother Tetsujin (effectively "Tetsujin Junior"). This new life embroils Kotetsu in the Syndicate’s criminal pursuits, which specifically target Miho.

Within Miho’s household, Kotetsu develops mutual romantic feelings for Tatsuya Mikado, a socially awkward ex-spy who abandoned his antagonistic past to join their ranks.

Beyond the original OVA, Kotetsu’s journey extends into an eroge visual novel sequel. Set shortly after the anime’s conclusion, it depicts her settled life at Miho’s residence alongside Tatsuya and allies. The game preserves the anime’s blend of modern and supernatural elements while incorporating explicit content absent from the source material.