At the dawn of the 21st century, the Kurobe Research Institute of the United Nations was established in Japan with the goal of analyzing an ancient artifact found during the construction of the Kurobe Dam. The organization attracted scientists from all over the world, whose children attended Mt. Tate International Senior High School. Yukina Shirahane, a shy student, happened to be the daughter of the institute's lead scientist. When she visited her mother, Yukina suddenly deciphered a fragment of the relic's riddle, causing a younger Samurai from the Sengoku era, Kennosuke Tokisada Ouma, to appear before her, startling her. As a celestial threat looms over the planet, Yukina and Kennosuke partner to ward off the invading army and protect Earth. While doing so, Yukina uncovers the mystery surrounding Kennosuke and his unyielding desire to safeguard her.