Tanjiro Kamado and Nezuko Kamado vs Swamp Demon focuses on the battle between demon slayer Tanjiro Kamado and demoness Nezuko Kamado against the Swamp Demon.
Every night, in the northwestern town, another young girl disappears. One night, Kazumi and his fiancée, Satoko, are walking around town and suddenly his fiancée disappears. He is held responsible for his sudden disappearance, but rumors circulate that a Demon lurks at night.
Tanjiro of the Demon Slayer Corps receives his first mission: to go to town and hunt down this demon. He believes Kazumi’s story and the pair spend the day looking for clues. It is only that night that the Demon becomes active again.
The Demon uses his Blood Demon Art to drag another girl named Tokie from the village, into his swampy dimension. His scent grows stronger, so Tanjiro is able to follow him to the exact spot where the Demon is holding Tokie underground. He drives his blade into the ground, forcing the pocket portal open. He spits out murky water, but Tanjiro sees Tokie’s kimono and can pull him out.
A Shapeshifting Demon appears from the bog and yells at Tanjiro before disappearing again. Kazumi catches up to him late and Tanjiro asks him to hold on to Tokie so he can protect them. Tanjiro examines the area and locations where the Demon might appear from. Even immersed in its own swamp, the Demon’s scent is not erased.
Tanjiro can sense the Demon ready to attack, he prepares the Fifth Form of Water Breathing: Blessed Rain after the drought as a parry and rushes to where the portal will appear. However, three puddles appear and three different Demons spring from the swamp. The young demon slayer jumps to avoid their grasp and calms down by changing shape. Tanjiro attempts to annihilate the three demons using Form Eight, Water Breathing: Waterfall Basin.
Unfortunately, because Tanjiro hastily changed his form, his attack was too shallow and he only manages to rip off the Demon’s arms. He calms down again, determined to make the Demon of Muzan Kibutsuji speak. The Demon attempts to go after Kazumi and Tokie, but Tanjiro shields them with Water Breathing, Second Form: Water Wheel.
Once again, Tanjiro’s attack is too shallow, but he was able to save the others. He lands two more quick hits at the demon and misses, forcing the demon to retreat. The young Demon Slayer cannot chase the Demon or do a full hit with Kazumi and Tokie nearby. The Two-Horned Demon is not amused and demands that the Demon Slayer back off or Tokie will lose his flavor. The Three-Horned Demon tells his other self to calm down and claims to have eaten a lot of sixteen-year-old girls already. Kazumi demands to know what the Demon did to his bride, only to find out that he devoured her.
Enraged, Tanjiro quickly dodges the Three-Horned Demon’s surprise attack. His arm splits the wall next to Tanjiro but he jumps on top of the Demon and cuts his arm. He considers that it is missed, then realizes that he approached too close to the wall. Tanjiro turns around to avoid the attack and is forced back by an attack coming from underground. The Two-Horned Demon almost gets hit while appearing from behind, but Nezuko comes out of her box and breaks the Demon’s neck.
The Swamp Demon asks why a swordswoman accompanies a Demon. The broken two-horned individual mends its neck and they all disappear into the bog. Under Sakonji Urokodaki’s hypnotic suggestion, Nezuko regards all humans as her family, and she will fight to protect them. She rushes on the Demons coming out of the swamp and tries to crush them with a violent blow of her axe.
The Demon disappears into the swamp in time to escape his attack. They try to pull her away but Tanjiro asks her to go back to his side. She runs towards him and one of the Demons tries to grab her, but she easily knocks him down to avoid it. Tanjiro then realizes he can let Nezuko protect Kazumi and Tokie while he concentrates on attacking.
A swamp portal opens below them as a diversion while the Two-Horned Demon goes after the girls. Nezuko protects Kazumi and Tokie and fends off the Demon. Tanjiro asks Nezuko to continue protecting Kazumi and Tokie while the young Demon Slayer goes underground. He descends into the Demon’s bog dimension, where there is almost no air and where the clothes of many victims are floating.
The two Demons inside the swamp think it must be hard for Tanjiro to move or breathe, and that he’s a fool for diving into it. They swim towards him at high speed, but the young demon slayer is not to be underestimated. At the top of Sagiri Mountain, where Tanjiro has spent more than two years training, the air is much thinner than inside the swamp. There, Tanjiro also learned the forms of Aquatic Breathing which can only unleash their full potential underwater.
The Demons pull away at a sharp angle just as Tanjiro is about to counterattack, but he knows he can just wait for them to close in again. The Demons attempt to pursue him from a different angle, but Tanjiro can sense the threat opening up. He uses a technique that can be triggered without any certainty, a fierce, cutting whirlwind created by a violent twisting of his upper and lower body; it is Water Breathing, Sixth Form: Whirlwind.
The sharp whirlwind slices the Demons to pieces, killing them instantly. Despite his victory, Tanjiro is saddened by the number of casualties the Demons have taken. The boy begins to run out of breath and swims back towards the entrance to the swamp. Meanwhile, Nezuko subdues the last remaining Demon with her bare hands.
Nezuko sneaks through four of the Demon’s strikes in a row, including a powerful hammer fist that crushes the ground. She follows up with a powerful punch to the Demon’s gut which is strong enough to make her cough up blood. The Demon manages to dodge the next punch, but he immediately gets knocked out by another punch and kick combo. The Demon attempts to slip into the bog again, but the girl acts quickly and kicks sweepingly so the Demon cannot escape. She tries to hit him with a flying kick but he is able to block it.
Nezuko is too fast for the Demon to dive back into the swamp, but he gets used to his direct fighting style. They exchange a few blows which they are able to redirect or avoid until the Demon narrowly blocks another blow to the head. He is able to block her next combo, confident that he can regenerate from whatever she throws at him. The Demon lunges at Nezuko and manages to block his jab, which gives him the opportunity to slash his forehead with his claws.
The Swamp Demon attempts to sink its claws into Nezuko’s face, but Tanjiro emerges from the portal in time to save his sister. Tanjiro cuts off the Demon’s arm, demanding that the Demon not raise a hand on his beloved sister. He then slices off the Demon’s other arm before he can attack again, and eventually defeats the Demon.
Continuation of the fight
The last Swamp Demon collapses and realizes that his fellows have been killed. Tanjiro tells him that he gives off a terrible stench and asks him how many girls he kidnapped. The Demon attempts to defend his actions by claiming that he was killing girls so they wouldn’t age and taste bad. Irritated, Tanjiro cuts out the Demon’s tongue and moves on to the next question.
Tanjiro presses his blade against the Demon and forces him back against a barrier. He demands to know all the information the Demon has about Muzan. However, Tanjiro is unprepared for the Demon’s fear of Muzan. He cowers and shakes to the bone, saying he can’t say anything.
The smell of fear is extremely strong and the Demon acts out of panic. He pushes his arms away and goes on a rampage, causing Tanjiro to chop off his head and finish him off. Tanjiro laments that he couldn’t gather any information and checks in on Nezuko. She fell asleep to heal her wounds, so Tanjiro promises to protect her from now on and puts her back in her box.