Lamenting about those who gave their lives in the continuous war against demon king Muzan Kibutsuji and his army, he declares that the human spirit will always rise to the challenge.

Directly after, all of them except for Nezuko fall into a deep sleep, the work of Enmu, Lower Rank One of Muzan's most powerful demons, the Twelve Kizuki.

Enmu instructs four passengers, all suffering from severe insomnia, to enter the Demon Slayers' dreams and destroy their spiritual cores to cripple them.

During their sleep, the Demon Slayers have idealistic dreams; Tanjiro reunites with his deceased family, Rengoku reminisces on his past with his younger brother Senjuro, and their alcoholic father Shinjuro.

Tanjiro and Inosuke find Enmu's neckbone in the locomotive but are caught off guard by its defenses, including a Blood Demon Art which constantly puts them to sleep.

Suddenly, they are attacked by Upper Rank Three of the Twelve Kizuki, Akaza, who tries to persuade Rengoku to turn into a demon and become immortal, after sensing his already immense power.

Akaza escapes nonetheless as Tanjiro breaks down, calling him a coward, and declaring Rengoku the victor of their duel because he fulfilled his duty to keep everyone safe.

In his last moment, Rengoku tells Tanjiro to visit his family's estate in order to ascertain writings from the previous Flame Hashira, which may inform his quest to learn more about his Hinokami Kagura technique.

He also encourages Tanjiro and his friends to continue on their paths to strength, telling them to never give up in protecting others, before succumbing to his injuries and reuniting with his mother in the afterlife.

While Tanjiro and his friends mourn Rengoku's death, the remaining Hashira receive the news, and Kagaya honors him for not letting a single passenger or comrade die in his presence, stating that he will be glad to reunite with him when he finally passes.

[18] On April 26, 2021, two months before its official release, the film was accidentally made available for purchase on the PlayStation Store for a few hours before being removed, resulting in leaked copies being distributed across the Internet.

[19][20] Upon its video-on-demand (VOD) release in North America, the film debuted at number one on the Vudu, Google Play and YouTube charts.

[22] The first part of the second season of the anime television series, subtitled Mugen Train Arc, is an extended and recompiled version of the film that ran for a total of seven episodes.

[41][42] It became the highest-grossing film of all time in Japan in 73 days at a gross of ¥32.48 billion, surpassing Spirited Away, which had held the record for 19 years.

The site's critics consensus reads, "Demon Slayer's visually stunning animation and masterful action set pieces serve a heartfelt plot that is sure to satisfy fans.

[66] Demon Slayer: Kimetsu no Yaiba the Movie: Mugen Train was one of the Jury Recommended works at the 25th Japan Media Arts Festival in 2022.

[70][71] Harding, Ehrlich, Morrissy and Debruge all noted that fully understanding and appreciating the film requires having watched the first season of the anime beforehand.