Most of the Kamado family has been slain by a Demon, and Tanjiro discovers that his sister Nezuko is only other survivor. He tries carrying her down the mountain to find a doctor, but she awakens and shakes them off the path. They fall into a snow-laced forest where Tanjiro discovers that Nezuko has become a Demon despite being born human.
Nezuko tries to feed on Tanjiro but he stuffs a hatchet in her mouth as she pins him to the ground. Her Demon form grows in size but Tanjiro pleads with Nezuko until her humanity cries out, spilling tears onto Tanjiro's face. Just when the skirmish appears to be over, a mysterious hunter appears and tries to slay Nezuko with a katana. Tanjiro moves her out of the way in time and only his hair is partially sliced off.
The hunter questions Tanjiro and quickly takes Nezuko hostage. The hunter explains he's a Demon Slayer and is going to kill Nezuko. Tanjiro begs him not to, asking him not to take away the last family member he has left. Annoyed by Tanjiro's pathetic groveling, the slayer gets angry and starts to berate him. He tells Tanjiro his family was massacred because he was weak and groveling will never work with Demons. In reality, the hunter blames himself for not getting to the mountain in time to save the humans. He's yelling at Tanjiro to help mold his resolve so he can eventually save his sister and avenge his family.
The Demon Slayer stabs his sword into Nezuko, forcing Tanjiro to act by throwing a rock his opponent is forced to deflect. Tanjiro grabs his hatchet and circles his target, using the trees as cover. He throws another rock while passing by a tree but the hunter easily dodges the stone.
Tanjiro charges straight for the hunter, and the latter believes this is a frontal assault driven purely on emotion. Disappointed, the hunter knocks out the boy using the hilt of his sword. After knocking Tanjiro down, the hunter notices that his hatchet is missing. He looks up and is nearly decapitated by the weapon flying through the air.
Giyu figures out that Tanjiro must have launched the hatchet at the same time as when he threw the second stone. He hid his hands as he pretended to attack to fool Giyu into thinking he was unarmed. Unlike before, the Demon Slayer is impressed by the boy's resolve to save his sister, attempting to bring down his opponent after falling in battle.
Nezuko breaks out of Giyu's hold and kicks him away. She goes over to Tanjiro, forcing Giyu to believe she'll try and devour him. However, Giyu is shocked when Nezuko protects her brother despite becoming a Demon. The hunter ponders Nezuko's state of mind as he dodges an assault from the newly-turned Demon. He keeps her back using only his hands and surmises that because she decided to protect her brother rather than eat him, these siblings must be truly unique.
Nezuko jumps off a tree and tries to attack Giyu's head. He sheathes his sword and uses a strong chop to knock her unconscious.
Tanjiro eventually wakes up to find that he and Nezuko are both unharmed. Giyu tells him to find a man named Sakonji Urokodaki who lives at the foot on Sagiri Mountain. Before disappearing, Giyu tells Tanjiro to keep Nezuko out of direct sunlight.
Tanjiro returns home to bury his family and says one final goodbye. Afterward, Tanjiro and Nezuko begin their long journey to return her to normal.