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 Demon Slayer: Mugen Train Arc TV series includes an all-new episode 1 featuring the day leading up to the events in the movie, plus re-edited 6 episodes featuring new scenes and music.

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Prequel: Kimetsu no Yaiba
Summary: Gekijouban Kimetsu no Yaiba: Mugen Ressha Hen
Sequel: Kimetsu no Yaiba: Yuukaku Hen