In the early 21st century, the United Nations Kurobe Research Institute in Japan delves into an ancient relic unearthed near the Kurobe Dam. While global scientists explore this artifact, their children lead regular lives at Mt. Tate International Senior High School. Yukina Shirahane, the quiet daughter of the institute's chief scientist, stumbles upon a clue to the relic's enigma during a visit. To her amazement, this triggers the appearance of Kennosuke Tokisada Ouma, a young samurai from the Sengoku period. With an extraterrestrial threat looming, Yukina and Kennosuke unite to safeguard Earth. Throughout their battle, Yukina unravels the secrets surrounding Kennosuke and his unwavering determination to protect her.