This story is about two childhood friends, Sayaka and Mushin, who have both practiced kendo for a long time. Sayaka is fairly strong and Mushin is not, but he dreams of protecting her anyway. Even though he doesn't realize it, Sayaka is the one who is always protecting him whenever he is in danger, without him being aware of it…. However, Mujin has a strong weapon of his own, which is something that has conquered even Sayaka: his earnest honesty and unfazed good will.

Cue to rivals popping up constantly, "evil organizations" trying to destroy Sayaka's and Mushin's Dojo, and even several love rivals who want to destroy their closeness, who are simply attracted by either Sayaka's charisma or Mushin's kindness…. Thus, both Sayaka and Mushin have their own battles and at times they may need to cooperate to surpass the impossible hurdles, of which being step-siblings is the very first one.