~Beyond the loneliness, there was a girl's feelings...~ In a remote land, a girl named Yona lives with her older brother in a sturdy fortress. Yona, who possesses mysterious powers, has shut her heart off from the world and lived alone with her brother in the "Isolated Fortress." Every day, agents seeking Yona's power visit this fortress, but they are driven away by her brother, who protects his sister, leaving without taking her. Then one day, a boy appears at the isolated fortress. Yona is bewildered by this encounter with the first person who opens his heart to her. Her brother, determined to protect his sister, attempts to eliminate the boy. Advising the brother is a mysterious monster... The boy, disregarding the danger, tries to take Yona away. At that moment, the girl is about to step onto the staircase toward the future, beyond her fleeting feelings.