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.
 
(Source: Crunchyroll)
 
Note: The first episode is entirely anime original. Episodes 2-7 are a re-edited version of the movie featuring new scenes and music.