Since the Taisho era, demons that have emerged for unknown reasons since the Heian era have continued to grow in power by devouring humans. The Demon Slayer Corps is the organization that hunts these demons. There are ten ranks, and above them are the Hashira, the highest-ranking combatants. Under the former Water Hashira, Urokodaki, the 13-year-old Guest, Tomioka Giyu, and Sabito trained hard to join the Corps, but Guest was unable to attend the Final Selection due to illness. The other two went, but Giyu was the only one who survived and passed. Giyu blamed himself for Sabito's death, believing Sabito died trying to protect him and save others. Five years later, Guest has grown into a young woman. A year after passing the Final Selection, Giyu, now the Water Hashira, takes Guest as his Tsuguko, or apprentice. Seeing each other for the first time in five years, Giyu and Guest feel a sense of awkwardness, as they are no longer the children who laughed together but a man and a woman with changed hearts and bodies. Living together, their feelings for each other grow. On the night Guest is promoted to Hashira a year later, they finally confess their hidden feelings. However, due to a sudden mission, Guest has to leave that very night for three years without even getting to say a proper goodbye to Giyu.

Now 21 years old, you have finally returned. As soon as you arrive, you are sent on another mission. Himejima Gyomei tells you that two Hashira went on a mission but haven't returned after two days, and asks you to go check on them. Thus, you head to Mount Natagumo, unaware of what might happen there.

After climbing Mount Natagumo for a bit, you see human silhouettes. Is it someone who got lost? Or perhaps one of the two Hashira? As you move closer, you realize there are two people. Specifically, the two Hashira: Tomioka Giyu, the Water Hashira, and Kochou Shinobu, the Insect Hashira. Tomioka Giyu is holding Kochou Shinobu in his arms. Even in the dark of night, the sight is clear.