Episode 3 of the series begins with a narrator explaining that Demon Slayers, who have existed for centuries, are not officially recognized by the government. The identity of the leader of the Demon Slayers is a closely guarded secret. Demons, who possess great power and feed on humans, can also increase in size. However, they can only be killed by sunlight or special swords.
Meanwhile, Tanjiro is training under Urokodaki with the goal of joining the Demon Slayer Corps. He undergoes rigorous daily training in preparation for the final selection process.
During his training, Urokodaki imparts important knowledge to Tanjiro about the sword. He explains that while the sword is powerful, it can weaken and break over time. Therefore, Tanjiro must apply strength appropriately with the right sword. Tanjiro continues his training with the sword and eventually learns the Water Style Swordsmanship technique.
Nezuko, Tanjiro’s sister, has been in a deep sleep for over six months. Concerned, Urokodaki calls a doctor, but there is nothing medically wrong with Nezuko. She is simply healing. Tanjiro expresses his worry, realizing that Nezuko cannot wake up in just one day, and fears that she may be dead.
The next day, Tanjiro goes for training in an area with even thinner air, making the training more challenging. Despite the difficulty, he successfully overcomes all the traps. After a year of intense training, Urokodaki informs Tanjiro that he has taught him all the necessary techniques and that further improvement is now up to Tanjiro’s dedication.
Urokodaki asks Tanjiro to follow him to a large boulder. Tanjiro is astonished by its size. Urokodaki challenges him to slice the boulder with his sword, stating that if he succeeds, he will be allowed to enter the Final Selection—an event where only those who survive become official members of the Demon Slayers Corps.
Tanjiro initially believes it is impossible to break such a massive boulder with his sword, as doing so would surely damage it. Urokodaki leaves, never to teach him another lesson.
Despite his doubts, Tanjiro attempts to cut the boulder every day using the techniques he learned. However, the stone remains unyielding. Frustrated, he starts doubting himself and sees himself as a failure.
After six months of relentless effort, Tanjiro still cannot make a dent in the rock. Meanwhile, Nezuko continues to sleep.
On the verge of giving up, Tanjiro notices a boy named Sabito, wearing a fox mask, sitting on the boulder. He wonders how Sabito arrived there and why he is wearing a mask.
Suddenly, Sabito attacks Tanjiro, inviting him to fight. Despite Sabito wielding a wooden sword, Tanjiro feels it would be unfair to fight an unarmed opponent. Despite his real sword, Tanjiro is at a disadvantage. Sabito taunts him, claiming that Tanjiro cannot harm him. Sabito reveals that he was the one who sliced the boulder and easily defeats Tanjiro in their duel.
Sabito also criticizes Tanjiro’s grasp of the Total Breathing technique, stating that he has merely memorized it without understanding its true application. Sabito strikes Tanjiro, knocking him down.
Later, Tanjiro wakes up at night to find a girl named Makomo, also wearing a fox mask, by his side. Makomo asks if he is okay, to which Tanjiro responds by expressing his admiration for Sabito’s attack and his desire to learn it. He asks Makomo if he can accomplish the same feat.
Makomo reassures him, promising to support him in his endeavors. She explains to Tanjiro the significance of Total Concentration Breathing, which increases blood circulation and raises body temperature, enabling humans to defeat demons. It is crucial to expand lung capacity to strengthen bones and muscles.
With Makomo’s guidance, Tanjiro learns techniques such as Total Concentration Breathing, which enhances his physical strength.
Makomo reveals that both she and Sabito were orphans raised by Urokodaki.
Despite extensive training, Tanjiro is still unable to defeat Sabito. Sabito occasionally challenges Tanjiro to fights, and each time, Tanjiro loses.
After six months of intense practice, Tanjiro faces off against Sabito once again. This time, Sabito wields a real sword. Tanjiro takes a deep breath and launches an attack at Sabito.
To Tanjiro’s surprise, his blade reaches Sabito first.
Sabito’s mask gets torn, revealing his smile. The adopted siblings vanish, leaving Tanjiro to realize that his sword, which he believed had cut through Sabito’s mask, had actually cleaved the boulder in two.
And with this revelation, the episode comes to a close.