Vivio Takamachi, a descendant of the Sankt Kaiser lineage in ancient Belka, once endured hardships but now enjoys a tranquil life surrounded by loved ones and engaging in martial arts training under the guidance of Nove Nakajima. Unexpectedly, Vivio becomes the focal point of a mysterious combatant, Heidi E. S. Ingvalt, who pressures Nove to disclose the whereabouts of the ancient Belkan royals. Refusing to cooperate, Nove faces a daunting challenge, only for Heidi to collapse suddenly. Unveiling her true identity as a middle schooler named Einhald Stratos, Heidi shares her peaceful intentions, aiming to showcase her martial prowess inherited from her ancestor, Claus Ingvalt, an ancient Belkan sovereign. Moved by Einhald's story, Nove arranges a meeting between Einhald and Vivio, leading to an unconventional yet promising bond. Through their training sessions and shared experiences, Vivio and Einhald forge a strong friendship, each finding solace and growth in the other's companionship.