High school girl Hayakawa Yui (Kuroshima Yuina) lacks motivation, sleeps in class, has no interest in finding a boyfriend, and is a big eater. Her only advantage is that she can run fast. Her younger brother, Tsutomu (Shimoda Shodai), is the opposite of her. He is a weak and talented student. He converted the warehouse into a laboratory and invented a machine that can travel through time and space. Yui accidentally activated the time machine and traveled to the Warring States Period, surviving as "Yui's helper". She met the young master of Kuroyu Castle, Yuki Kyuhachiro Tadaki (Kentaro) and fell in love with him. Yui decided to accompany him as a small soldier to change the historical fate of the young master's death.