Giyu Tomioka: Gaiden (Part 1) (冨岡 義勇: 外伝 Tomioka Giyū: Gaiden?) is the first of a two part side story based on an encounter of the Water Hashira, Giyu Tomioka. It takes place sometime after he first meets Tanjiro Kamado and his sister.
The gaiden is a crossover project written by the Kimetsu no Yaiba author, Koyoharu Gotōge, and illustrated by Kimetsu no Aima! author, Ryōji Hirano. It is the first spin-off series of the Kimestu no Yaiba franchise.
The story opens onto Giyu Tomioka standing in front of a Demon, who is clutching a woman he had been midway through eating. The Demon begs Giyu to spare him, but, as he draws his Nichirin Blade, Giyu responds that they both know him begging for his life is pointless. Panicking, the Demon drops the woman's body and starts to back away from Giyu, then lunges towards him screaming that he swears he'll never eat a human again. With one swift movement, Giyu beheads him with a Water Breathing style.
Just as he begins to put his blade back in its sheath, a Kasugaigarasu crow flies overhead, telling Giyu to head to the snowy mountains and investigate the recent deaths that have occurred there due to a Demon.
Meanwhile, at the Northern Rest Point, Shinobu Kocho is conversing with an old woman about a "man-eating bear" that had been eating many of the Matagi mountain hunters from their village. Shinobu says it's no wonder, since she'd been feeling that the air around that rest point had felt quite tense. The woman tells Shinobu that she should head back quickly, but the Hashira explains that she's actually free right now, and that dealing with a "bear" that has eaten humans is very much within her sphere of work anyway.
Shinobu asks the woman if there's anything else she can tell her about the subject, but a cry from outside cuts her off before she can say anything. A man from the village is shouting at a young girl for going into the mountains alone again, and the girl is protesting that she wasn't alone because she had Taro, her dog, with her. Shinobu asks what's going on, and the woman explains that the girl's name is Yae, the daughter of a Matagi hunter named Matazo. The girl had apparently witnessed her father and friends's death after they'd been attacked by the rumoured bear, and ever since then she'd been heading into the mountains to attempt to seek vengeance.
Shinobu watches, shocked, as the gun hanging from Yae's back is suddenly grabbed by a man who's approached her from behind, and the girl whips around, shouting at the man not to touch her gun and attempting to deliver a blow to him with her right elbow. Her arm is stopped by the man, who is revealed to be Giyu.
Giyu begins to introduce himself, saying he's with the Demon Slayer Corps and requesting that Yae tell him what she knows about the Demon that had been rampaging around the village. Confused, the nearby villagers mutter that it was a bear, not a "Demon," and ask what he's talking about. Shinobu knocks her head onto the wall in frustration. One villager asks Giyu if he's a policeman, but he says again that he's a Demon Slayer, to which the villager asks him to his denial if he's a political official. He's asked about the Nichirin Blade hanging at his waist, which he produces, lying it on the floor and introducing himself once again as a Demon Slayer as he gets down on one knee. Worried, the villagers ask each other if they should call the police, and then one pipes up, saying that Giyu could be the culprit responsible for murdering a family of coal-burners since the prime suspect for that case also wore a half-and-half haori.
Shocked, one villager points a gun at Giyu as others fetch rope to tie him up with, and that's when Shinobu begins to approach the scene. She walks up behind Giyu, poking his arm and exclaiming what a coincidence it is for her to see him out here. She says she's glad to see that he's still awful at talking, and offers to explain the situation for him, but Giyu says he can handle himself, as the villagers begin tying him up.
Yae starts to speak, saying that she doesn't know what he wants, but then falls to the floor before she can finish her sentence. Shinobu inspects her, saying she has a fever and asking Giyu to help her out. Giyu breaks out of the ropes that had been binding him, asking if she's sure, to which she replies that she might as well help out since Demon extermination is her job, after all.
While unconscious, Yae starts to dream of her childhood. She's playing outside with her dog as her father and another man discuss indoors the fact that Yae wants to become a Matagi. Her father, Matazo, says that he told her hunting is a game and that he's never heard of a female Matagi before, but the other man argues that Yae must be lonely at home since there aren't any other kids for her to play with. He says that she must just want to be with Matazo, asking him to understand her feelings.
Matazo exits the house and walks up to Yae and Taro, saying that if she messes around with the gun he's taking is back, and Yae smiles widely.
Yae's childhood flashback ends, and another begins, showcasing the time Yae found her family and friends had been murdered. She gasps, asking how and why this could happen, and then looks up and sees someone eating one of the bodies, calling "Father?" out to him. Yae and her father's eyes meet, but all she sees is a man-eating bear.
When Yae awakens, she finds herself in a bed in the pharmacy on the other side of her village. She opens her eyes to find Shinobu sat close to her, bending down with her face near hers, and she blushes and cries out when she sees her. Yae sits up and moves away, clutching her gun, and Shinobu laughs, asking her to please not shoot her. Shinobu explains the situation, telling Yae her location and the fact that she was unconscious for half a day, then offering her herbal medicine and saying that she can't avenge anyone while in her current state. Yae takes the medicine, asking what's in it, but Shinobu just tells her to drink it.
Meanwhile, in the other room, Giyu is sitting with Taro. He reaches out to pat the dog, but he bites down on his hand, not letting go even when Giyu panics and starts to shake it. As Giyu and Taro struggle further, Yae calls out to her dog, telling him to stop it and apologizing, since he's usually good with people. Shinobu says not to worry because Giyu is just one of those people.
Shinobu then turns to Yae, offering her condolences for the girl's father's passing and asking her if she could question her about the incident. She explains that whatever attacked them could be a Demon, and then Giyu, Taro still attached to him, interrupts and says that he should do the explaining. Shinobu reminds him that he's being attacked, and, after some protesting from him, Shinobu gets Giyu to back off.
Yae and Shinobu's conversation continues, and, after telling Shinobu what she knows about the bear, Yae says it's her turn to ask questions. She asks what a Demon is and what Demon Slayers are, to which Shinobu explains the basics of what makes a Demon, including the regeneration, strength, lifespan and the two ways to kill it, and the Demon Slaying Corps that purge Demons from the world. Yae asks if they slaughter Demons, and Shinobu tells her not to phrase it like they're executing people since all they're doing is ridding the world from evil Demons.
Shinobu then says that she must have been wrong about Yae's case, since the attacker seems to genuinely be a bear, but still offers her her's and Giyu's help to hunt the bear. Yae simply says that she's a Matagi, so she can do her own avenging.
Yae then gets ready to leave, thanking Shinobu for taking care of her and ignoring the Hashira's protests that she still needs to rest. As she makes her way towards the door of the pharmacy, Giyu walks over to her and stand before her. He advises her not to walk around at night, but Yae hot-headedly replies that there aren't any Demons around and there's nothing wrong with a Matagi shooting down a pest. Giyu asks her why she's going into the mountains to avenge someone and asks her if she's hiding something from them. Yae shouts out that he's annoying, and pushes past him to get to the door. Just before she's gone, Giyu tells her that, at the very least, he's never seen a Demon turn back to a human.
As Yae ventures out onto the snowy mountains with Taro, crouching down and paying her respects at the pile of sticks that used to be her home, Shinobu begins to discuss the case with Giyu through the wall dividing the the two rooms, Shinobu bringing up the subject of "den-deprived" bears that are unable to find a den in the winter and so aren't able to go into hibernation. She calls them sorrowful creatures, lacking a home or family, who prowl the mountains at night. Shinobu then opens the door partitioning the rooms, appearing dressed in her corps uniform, and tells Giyu that if they leave her alone, she won't be saved.
In the meantime, Yae and Taro keep walking. Yae tells her dog to follow the smell, crouching down next to him, stroking him and telling him to keep pushing despite the freezing cold. Yae remembers her father again, and the time that he spent teaching her the skills needed to become a Matagi. They were sitting next to a bear they had just taken down, and her father was telling Yae never to waste a part of a bear because every part was a blessing of the mountain. He says that they as Matagi live off what the mountain gives them, and that, because they kill, they have to live.
The flashback ends, and Taro begins barking at something, running off ahead of Yae to investigate it. She calls after him, and then sees something fly backwards towards her and fall onto the snow. Shocked, she realizes it's Taro's decapitated head, and moves on forwards to reach the top of the hill. On the other side, she sees a figure. Yae knows immediately that it's the man that was once her father, bent over the corpse of Taro, and thinks to herself that he really did become a Demon.
Readying her gun, Yae remembers the shooting lessons her father gave her, getting into position to fire at him. She thinks about how she wanted it to be a mistake, and the fact that she lied about her Demon father being a man-eating bear, but Taro's death was the last straw. Yae pulls the trigger, blowing her father's head off. As the Demon begins to recover, regenerating his head and running towards her, she asks in her mind how he became a Demon and why he had to kill everyone. Aiming her gun at him again, she silently asks why he fled from their house like he was running away.
Matazo jumps on Yae, pinning her down, and she slams her gun side-on in his mouth to gag him. Yae thinks that she will shoot him if he can't turn back, and then asks him aloud, as she begins to cry, why he didn't eat her as well that day. The Demon begins to say something, and suddenly is leaped on from behind. Giyu appears, swinging his sword at the Demon, who leaps away from Yae, one arm severed. He stands between Yae and the Demon, Shinobu standing behind them both and whispering to Yae over her shoulder what a coincidence this was.