The Brave 'Yoshihiko'(勇者ヨシヒコと魔王の城)

The story is set in an ancient time, when the whole country was covered in thick forests. A mysterious plague breaks out, resulting in countless deaths. A pure young man named Yoshihiko becomes chosen as a hero, and he sets out on a journey in order to save a village. He has two goals – to bring back the legendary plant that can cure the plague, and to rescue his father Teruhiko who left to find the plant but never returned.