Four-year-old Kun is jealous of his new baby sister until one day when he storms off into the garden and meets her as a teenager from the future.

It's 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. Though devastated by this grim reality, Tanjiro resolves to become a demon slayer to turn his sister back and kill the demon that massacred his family.