From MangaHelpers:

Nijou Hanabi (girl) and Natori Idzumi (boy) are headed for their first day of middle school. Hanabi and Idzumi have spent most of their lives together; their parents were neighbors and close friends, they've learned kendo together, they've always been in the same classes at school and were even seated near one another. When their homeroom teacher tells them that middle school will be different from elementary school, Hanabi can't really see it at first. She's seated next to Idzumi, as always, and they are still as close as ever. The two of them begin to meet new friends and acquaintances, and they, along with their new friend Homare, join the kendo club. However, Homare and Hanabi are the first and only females in the club, so they get a bit more attention than they would have expected. From their first club meeting, Hanabi's life begins to be more complicated, as barely understood jealousies begin creeping into her relationship with Idzumi. It seems that becoming a teenager is going to be very different than being a child! [tethysdust]