NOTE: This spoiler was submitted by Jeremy

Brief show recap for context: Tanjiro Kamado trains to become a Demon Slayer after his whole family is slain by a demon and his younger sister Nezuko is turned into one. They have managed to keep her bloodlust under control, preventing her from attacking humans and maintaining her strength through rest, and he carries her on his back in a box since sunlight kills demons. They team up with the cowardly Zenitsu Agatsuma and hotheaded Insouke Hashibira, who wears a boar’s hide as a mask as they seek out other demons sent by top demon Muzan Kibutsuji, called the Twelve Kizuki. After the top Demon Slayers, the Hashira, determine that Tanjiro and Nezuko may live despite the conflict of a human working with a demon, and they are instructed to board the Mugen Train to catch another demon after it is reported that multiple Demon Slayers have gone missing from there.

The film opens with Hashira Master Kagaya Ubuyashiki walking through a cemetery with his wife Amane, reciting the names of fallen Demon Slayers. He feels a sense of responsibility for sending so many of them, as young as children, off to their deaths in fighting powerful demons.

Tanjiro, Nezuko, Zenitsu, and Inosuke board the Mugen Train, with Inosuke thinking that the train is a great beast since he has never seen one before. They meet with one of the top Hashira, Kyojuro Rengoku, to catch the demon. Tanjiro talks to Kyojuro about the Hinokami Kagura, a fire technique that his father Tanjuro once knew. Moments later, a large demon (but not their target) attacks the train, prompting the Demon Slayers to protect the other passengers. Kyojuro uses a powerful flame technique with his sword and manages to vanquish the demon.

The main demon, a Lower One Twelve Kizuki named Enmu, makes his way onto the train and puts everyone onboard the train to sleep. He then finds four children with insomnia and uses his disembodied hand to instruct them to enter the dreams of the Demon Slayers.

The four Demon Slayers are all fast asleep. Tanjiro finds himself walking in the snow near what was once his old home. To his surprise, he sees his younger siblings Hanako and Shigeru greeting him. He runs to them and begins wailing hysterically, thinking he failed to save them. He is then brought to his home, where he finds his mother Kie and other siblings Takeo and Rokuta all still alive. Meanwhile, Zenitsu is stuck in a dream where he is frolicking happily with a human Nezuko, while Inosuke dreams he and his companions (all appearing as goofy animals) are set to fight the spider-like train. Kyojuro sees himself with his family as well, such as with his unsupportive father Shinjuro, and his younger brother Senjuro, whom he always tried to encourage.

One of the minion children enters Kyojuro’s dream and finds the edge of the dream to find his spiritual core. Before she can destroy it, Kyojuro manages to grab her by the throat, even as he still sleeps, which weakens and prevents her from attacking. In Tanjiro’s dream, one of the children that has tuberculosis finds Tanjiro’s core, but is stunned to find how calm and serene the projection of Tanjiro’s heart is.

Dream-Tanjiro is told by his siblings that Nezuko has gone up the mountain to pick fruit. As he goes by the river, he sees his reflection pulling him underwater and telling him to wake up since he is trapped in a dream. After he goes back to his home, Tanjiro starts to realize that the world around him is false. Outside, Nezuko emerges from her box and tries to wake up her brother. She even headbutts him, which causes her to bleed and activates her Blood Demon Art, which Tanjiro feels in his dream. Knowing what he must do, he runs away from the family, even as he sees Human-Nezuko and Rokuta tearfully running toward him. Tanjiro sees his father, who tells him that there is something he must cut in order to reawaken. Tanjiro cuts his own neck and manages to wake himself up.

Tanjiro has Nezuko use her powers to sever the binds to the other Demon Slayers, but he still has to find a way to wake them up. The minion children rise up and antagonize Tanjiro because they know that Enmu won’t reward them with dreams if they fail. Tanjiro incapacitates the children and speaks to the boy who saw inside his heart, and he realizes that Tanjiro is pure. Tanjiro lets him go.

Tanjiro goes to the roof of the train while Nezuko tries to wake up the others. Tanjiro fights Enmu, who keeps trying to put him back to sleep, but Tanjiro keeps waking himself up by killing himself in his dreams. Enmu recognizes Tanjiro as the boy Muzan ordered him to kill so that Enmu may gain greater power from Muzan’s blood. He attempts to manipulate Tanjiro by projecting a dream of his family asking why he couldn’t save them, but it only enrages the boy. Tanjiro utilizes his special water dragon technique to decapitate Enmu, only for the demon to reveal that he has fused himself with the entire train, making the train a demon and keeping all 200 passengers as his hostages.

Inosuke and Kyojuro wake up while Nezuko fights off Enmu’s tentacles in the train. When they appear to overpower her, a still-sleeping Zenitsu uses his lightning technique to instinctively protect her. Inosuke joins Tanjiro as they try to look for the train car that connects to Enmu’s head. Tanjiro is attacked by the conductor briefly, but Inosuke incapacitates the man. Enmu has multiple eyes that cause Tanjiro to keep falling asleep, nearly tricking him into killing himself for real until Inosuke steps in and stops him. Tanjiro uses his sword to slice through Enmu’s attacks until he finally sees the neckbone in the train, using a powerful water technique to slice through and finally behead Enmu, which also causes the train to derail, but the Demon Slayers do what they can to protect the passengers.

As Enmu dies, he laments his failure to the whole Demon Slayer team as he slowly disintegrates. However, an Upper Three Twelve Kizuki, Akaza, emerges to battle the Demon Slayers. He acknowledges Kyojuro’s power and wants to turn him into a demon, but he refuses and battles Akaza. The fight rages on until Akaza punches through Kyojuro’s chest, with the only option for survival being for Akaza to turn him into a demon. Kyojuro remains defiant and starts to slice his sword through Akaza’s neck as the sun comes up. Kyojuro sees a flashback of his mother Ruka reminding him to fulfill his duties. Akaza manages to escape Kyojuro’s grasp as the sun comes up, and Tanjiro attempts to send an attack his way, but Akaza has escaped.

Tanjiro goes to Kyojuro’s side as he is dying. He tells Tanjiro that he has faith in him, and that he and Nezuko are more than capable of working together against more evil demons. In his final moments, Kyojuro sees his mother again, who tells him that she is proud of him. Tanjiro weeps for Kyojuro, but Inosuke tells him that they need to go train since Kyojuro just expressed his encouragement to the boys. However, the two of them, plus Zenitsu, take a moment to mourn their fallen friend.

The messenger crow reports the news of Kyojuro’s death to the other Hashira. Master Ibuyashiki laments the loss but is thankful to the Demon Slayers for protecting the passengers. He then tells Amane that he may soon join Kyojuro in death.

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The Demon Slayers - Tanjiro, Nezuko, Zenitsu, and Inosuke - board the Mugen Train to catch a demon that has targeted other Demon Slayers. They team up with high-ranking Hashira Demon Slayer Kyojuro to fight, only for the demon, Enmu, to trap the Demon Slayers in their dreams and have four minion children enter the dreams to kill them.

Nezuko (who is a demon that protects humans) wakes up Tanjiro (her brother) so he can fight Enmu while she wakes up the others. Enmu reveals he fused himself to the train and is keeping all the passengers hostage. Tanjiro and Inosuke fight Enmu outside while the other three fight inside. Tanjiro manages to decapitate Enmu and kill him, but a stronger demon named Akaza comes out to challenge Kyojuro and turn him into a demon. Kyojuro refuses and fights Akaza, who fatally wounds him and manages to get away before they kill him. Kyojuro puts his faith in Tanjiro and his friends to continue carrying out the work of the Demon Slayers.