Vivio Takamachi, a Sankt Kaiser royalty of ancient Belka, was previously subjected to mistreatment. Fortunately, she now lives a peaceful life with family and friends while practicing martial arts with her coach, Nove Nakajima. However, Vivio soon becomes the target of the mysterious fighter Heidi E. S. Ingvalt, who demands Nove to reveal the location of the ancient Belkan royals. Refusing to disclose this information, Nove finds herself in a losing battle but Heidi soon falls unconscious. In addition, Nove discovers Heidi's true identity to be a middle school student named Einhald Stratos.
When she regains consciousness, Einhald clarifies that she has no animosity toward ancient Belkan royals. Having inherited the passion to defend and harboring regretful memories of her ancestor, the ancient Belkan ruler Claus Ingvalt, she simply wants to prove that her fighting ability is strong enough to protect others. Intrigued by the girl's situation, Nove arranges for Einhald to meet with Vivio. Although they get off to an unconventional start, Vivio enthusiastically trains with Einhald, assisting her in finding peace of mind while developing a vivid friendship along the way.