At the all-girls Tenchi Academy, there exists an exceptional class of students known as "sword-bearers", who regularly compete in school-sanctioned "hoshitori" duels to improve their skills and standing. Among them is Kurogane Hayate, who initially transfers in while masquerading as her twin Nagi until her sister can return to school. Although exposed almost immediately, Hayate decides to join the hoshitori in earnest after Dandelion Garden, the orphanage that raised her, is revealed to be in crippling debt. Hoping to earn enough prize money to repay it, Hayate becomes "shinyuu", or duel partners, with fellow student Mudou Ayana, and the two begin ascending the hoshitori ranks. They almost break up following Nagi's return, but manage to reconcile and successfully pay off the orphanage's debt.

During a showdown with Someya Yukari, Ayana's former shinyuu, to settle their differences, the Hayate-Ayana pair encounter a mysterious group of students calling themselves the Kurogumi. Led by Fubuki Hajime, President Hitsugi's younger cousin, the Kurogumi declare their intent to seize control of Tenchi Academy. Hitsugi welcomes the challenge, splitting the sword-bearing student body into the Heaven and Earth Dorms, or the Tenchi and Kurogumi camps respectively, to fight for the future of the school. Heaven or earth? Each girl must decide for herself which path to take, setting up for a clash that threatens to engulf the entire academy in stupid fun...