Tanjiro encounters Mist Hashira Muichiro Tokito and Kotetsu, a boy from the village, arguing. Muichiro demands that a "certain key" be handed over to him from Kotetsu. Being unable to look on in silence, Tanjiro intervenes to speak on behalf of Kotetsu. However, Muichiro refuses to listen. According to Kotetsu, the key is used to operate a battle-type mechanical doll, Yoriichi Type Zero, that mimics the movement of a certain real-life swordsman...