Short answers, pulled from the story.
Demon Slayer: Mugen Train follows Tanjiro Kamado, his demon sister Nezuko, and their companions as they board a train to assist Flame Hashira Kyojuro Rengoku in hunting a demon. The demon Enmu traps the slayers in enchanted dreams before merging his body with the train, and a subsequent battle with Upper Rank demon Akaza ends with Rengoku's death.
Demon Slayer: Mugen Train grossed over $512.7 million worldwide from more than 41 million tickets sold. It was the highest-grossing film of 2020 globally and the first non-American production in cinema history to top the annual worldwide box office.
Demon Slayer: Mugen Train premiered in Japan on the 16th of October 2020. In the United States and Canada, it opened on the 23rd of April 2021, where it grossed $47.7 million.
Haruo Sotozaki directed Demon Slayer: Mugen Train, having also directed the anime television series it follows. The film was produced by Ufotable, with Akira Matsushima as animator and Manabu Kamino as editor.
Demon Slayer: Mugen Train surpassed Spirited Away as the highest-grossing film of all time in Japan within 73 days, ending a 19-year record. It was also the first film to top the Japanese box office for ten consecutive weekends since weekly charts began in 2004, and remained in the top 10 for 32 consecutive weeks.
Demon Slayer: Mugen Train won Animation of the Year at the 44th Japan Academy Film Prize, Best Animation Film at the 45th Hochi Film Award, and Best Film at the 6th Crunchyroll Anime Awards. Composers Yuki Kajiura and Go Shiina also won Outstanding Achievement in Music at the Japan Academy Film Prize.