Every night in the town to the northwest, another young girl disappears. One night, Kazumi and his fiancee, Satoko, walk through town and suddenly the girl goes missing. He's blamed for her sudden disappearance, but rumors swirl that there might be a Demon lurking at night.
Tanjiro of the Demon Slayer Corps receives his first mission to head into town and hunt that Demon down. He believes Kazumi's story and the pair spend all day searching for clues. It is not until that night when the Demon becomes active again.
The Demon uses his Blood Demon Art to drag another girl into his swamp dimension. Its scent gets stronger, so Tanjiro is able to track it down to the exact spot the Demon is holding the girl underground. He stabs his blade into the ground, forcing the pocket portal to open. It spews out murky water, but Tanjiro sees the girl's kimono and is able to pull her out.
A morphed Demon appears from the bog and screeches at Tanjiro before disappearing again. Kazumi catches up to him late and Tanjiro asks him to hold the girl so he can protect them. Tanjiro examines the area and the possible places the Demon could appear from. Even when submerged in its own swamp, the Demon's scent isn't erased.
Tanjiro can smell the Demon ready to attack, he prepares the fifth form as a counter and rushes to the spot where the portal will appear. However, three bog puddles appear and three different Demons sprawl out of the swamp. The young Demon Slayer jumps to avoid their grasp and calms himself as he switches forms. Tanjiro tries to lay waste to the three Demons using Water Breathing: Eight Form - Waterfall Basin.
Unfortunately, because Tanjiro hastily switched forms, his attack was too shallow of a strike and he only manages to leave off the Demon's arms. He calms himself again, determined to make the Demon talk about Muzan Kibutsuji. The Demon tries to go for Kazumi and the girl, but Tanjiro protects 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 takes two more quick swings at the demon and misses, forcing the Demon to retreat. The young Demon Slayer can't chase the Demon or take a full swing with Kazumi and the girl nearby. The two-horned Demon isn't amused and demands the Demon Slayer back off or else the girl will lose flavor. The three-horned Demon tells his other self to calm down and claims to have eaten plenty of sixteen-year-old girls already. Kazumi demands to know what the Demon did with his fiancée, only to find out that they devoured her.
Enraged, Tanjiro quickly evades the three-horned Demon's surprise attack. Its arm cracks the wall next to Tanjiro but he jumps above the Demon and slashes its arm off. He considers this a miss and then realizes he's come too close to the wall. Tanjiro twists his body over to avoid the attack and is forced to back up by an attack from underground. The two-horned Demon nearly lands a hit by appearing from behind, but Nezuko kicks out of her box and breaks the Demon's neck.
The Swamp Demon questions why a swordsman would be accompanying a Demon. The broken two-horned self fixes its neck and they all fade back into the bog. Under hypnotic suggestion from Sakonji Urokodaki, Nezuko sees all humans as her family, and she will fight to protect them. The Demon girl rushes the Demons as they appear from the swamp and tries to crush them with a fierce axe kick.
The Demon disappears into the bog in time to evade her attack. They try to lead the girl away but Tanjiro asks her to return to his side. She runs back to him and one of the Demons try to grab her, but she easily flips over him to avoid it. This allows Tanjiro to realize that he can let Nezuko protect Kazumi while he focuses on attacking.
A swamp portal opens beneath them as a distraction while the two-horned Demon goes after the girls. Nezuko protects Kazumi and keeps the Demon back. Tanjiro asks Nezuko to continue protecting Kazumi while the young Demon Slayer heads underground. He descends into the Demon's bog dimension, where there is hardly any air and the clothes of many victims float about.
The two Demons inside the swamp believe it must be hard for Tanjiro to move or breathe, and that he's a fool for diving in. They swim at him at high-speed, but the young Demon Slayer isn't to be underestimated. On top of Sagiri Mountain, where Tanjiro spent over two years training, the air was much thinner than inside the swamp. There, Tanjiro also learned water breathing forms that can only unleash full potential under water.
The Demons dart away at a sharp angle just as Tanjiro is about to counterattack, but he knows he can just wait for them to get close again. The Demons try and rush him from a different angle, but Tanjiro can smell the opening threat. He uses a technique that can be unleashed with no sure footing, a fierce slicing vortex created by a violent twist of his upper and lower body; Water Breathing: Sixth Form - Whirlpool.
The slicing whirlpool slashes apart the Demons into pieces, killing them instantly. Despite his victory, Tanjiro is saddened by the number of victims that the Demons have claimed. The boy is running short on breath and swims back toward the entrance to the swamp. Meanwhile, Nezuko is overpowering the last remaining Demon with her bare hands.
Nezuko weaves through four of the Demon's strikes in a row, including a strong hammer fist that crushes the ground. She follows up with a powerful jab into the Demon's gut that's strong enough to make him cough up blood. The Demon manages to evade her next punch, but he immediately gets clobbered by another punch and kick combo. The Demon tries to slip back into the bog, but the girl acts quickly and sweep kicks so the Demon can't escape. She tries to hit him with a flying kick but he's able to block it.
Nezuko is too fast for the Demon to dive back into the swamp, but he gets used to her straightforward fighting style. They exchange a few blows that they're able to either redirect or avoid until the Demon narrowly blocks another strong to his head. He's able to block her next combo, confident he can regenerate from anything she throws at him. The Demon rushes Nezuko and is able to block her jab, giving him an opening to slash her forehead with his claws.
The Swamp Demon tries to finish off Nezuko by stabbing his claws into her face, but Tanjiro emerges from the portal in time to save his sister. Tanjiro cuts off the Demon's arm, demanding the Demon not lay a hand on his beloved sister. He follows up by slicing off the Demon's other arm before it can attack again, ultimately defeating the Demon.
The last remaining Swamp Demon falls over and realizes its other selves have been killed. Tanjiro tells him he gives off a terrible stench and asks how many girls he's kidnapped. The Demon tries to defend its actions by claiming he was killing girls so they wouldn't get old and taste bad. Irritated, Tanjiro cuts out the Demon's tongue and moves on to the next question.
Tanjiro presses his blade close to the Demon and forces him back against a fence. He demands to know all the information the Demon has on Muzan. However, Tanjiro is not prepared for the crippling fear of Muzan the Demon has. He cowers and trembles down to his bones, claiming he can't say anything.
The scent of fear is extremely strong and the Demon acts out of panic. He grows his arms back and lashes out, forcing Tanjiro to cut his head off and end him. Tanjiro laments at being unable to gather any information and checks up on Nezuko. She has fallen asleep to heal her wounds, so Tanjiro promises to protect her from now on and puts her back inside her box.