It is the Taisho Period in Japan. Tanjiro, a kindhearted boy who sells charcoal for a living, finds his family slaughtered by a demon. To make matters worse, his younger sister Nezuko, the sole survivor, has been transformed into a demon herself. Though devastated by this grim reality, Tanjiro resolves to become a “demon slayer” so that he can turn his sister back into a human, and kill the demon that massacred his family.

https://en.wikipedia.org/wiki/Hajime_no_Ippo