Flame Hashira Kyojuro Rengoku receives new orders: Travel to the Mugen Train, where over forty people have gone missing, and conduct an investigation. Leaving the Demon Slayer Corps Headquarters, he sets off on this new mission. Tanjiro's group, fresh from completing their training at the Butterfly Mansion, also head to their next mission, the Mugen Train. There they rendezvous with Rengoku, who arrived ahead of them. On this train running through the darkness, a new battle is about to begin.