Overview Musashi and Kanemaki Kojiro as children vow to become the greatest Bushi. Five years have passed and Musashi's determination is left intact albeit professes his desire to become a miner for the Oni, however Kojiro has lost his determination questioning his father's tale of the battle between Bushi and Oni. Those from Bushi blood, as is Kojiro, are shown to be marginalized in their society since Oni have ruled for 150 years.
Kojiro is visited by Musashi who seeks to leave town to become Bushi, claiming that he has been training during mining practice all these years with his pickaxe sword. Kojiro denies him saying he is untruthful of his intentions and dreams. When Musashi is at his ceremony to become a miner for Oni, he vows to show Kojiro of his seriousness by slaying an Oni. When the miners are shown the truth of their reality, Musashi tries to attack the Oni but is overpowered. Kojiro enters with Musashi's sword on a kitetsuki, and they attack the Oni reaffirming their desire once again to become the greatest Bushi together.
The story begins in Kiho Year 146 with Musashi and Kanemaki Kojiro at ten years of age sparing in a fencing match. The match between the two seems to be equal sided in it seriousness, however Kojiro says that Musashi's attacks are "boring," suggesting that he is more skilled. After dueling, Musashi urges that they look at Kojiro's father's picture scroll, which entails the struggling endeavor of the bold Bushi and evil Oni. The young boys are enthralled by the tale and the comradery of the warriors, and vow to become greatest Bushi themselves.
Five years have passed since that promise (both 15 years old), and Musashi is mining veraciously with a giant sword-like pickaxe. Musashi tells the audience that he is a miner and not a Bushi because "the world" — likely meaning Hinomoto — is ruled by, worships and protects the godly Oni. He says he rejects the belief of deity Oni and the town's ways of life. He is called a mining prodigy by his companions for his hard work and being the best miner. A teacher's shoutings of 'history' before the Oni's reign 150 years ago and Musashi's narrative show that Bushi are marginalized and slandered as "inhumane monsters" in their society. Musashi's mining class ends with a chant of their dreams, "to become a miner!" which Musashi hesitantly says thinking rather to become a Bushi, which thereafter he feels repelled that he can't display his true feelings.
Musashi goes to Kojiro's house, where Kojiro is smoking after working and tells Musashi to "go home." Musashi asks if Kojiro would like to get food, but Korjiro denies as he is conserving money. After Kojiro again tries to rid of him, Musashi shows his "pickaxe shaped sword" to him and says he has been practicing so he could leave town with Kojiro. Kojiro refuses, claiming that his dad came up with "pretty lies" about the Bushi. Kojiro accuses him for wanting to be a miner, not a Bushi, and lies about his true feelings, therefore says that he can't trust his word. Musashi storms out claiming that he will demonstrate his seriousness of becoming a Bushi to him. Now alone, Kojiro seems content with "his brother," saying that he wants him to "live an honest life" and "it's alright, [if he] become[s] a miner."
Two days later at the Tatsuyama Mine Mountain Entrance Ceremony, Musashi is still distressing with Kojiro's words. In order to prove himself to him, Musashi pledges to himself that he will take down an Oni here. When the officers reveal the "employee's" new work place and claim that they will be there till death, everyone is aghast by the hundreds of slaving miners and the slaves being killed by Oni. Musashi's classmates realize that they have been fooled of reality. Musashi proclaims he will slay them and grabs for his sword but realizes that it is not there as he left it.
At his home, Kojiro is recollecting their fun past until Musashi "gave up sword-fighting" but then says it better that he is a miner than to be affiliated with Bushi families. Kojiro notices the bag he left two days ago, he looks inside and finds his "Training Book."
Back at the slave mining field his friends try to persuade Musashi to stop while he is being thrashed by an Oni as he vainly endeavors to attack it with a tiny pickaxe. Musashi reflects on his deception of becoming a Bushi to his friends, saying he was fooling himself for being "afraid of rejection." He musters his strength and proclaims he wants to become a Bushi, when an officer prompts him of his dream seeking his desire to be a miner such that he could make the Oni stop attacking.
Kojiro enters and crashes into the Oni on his father's kitetsuki (as called by an officer for a motorbike-like object) temporarily saving Musashi in the process. Kojiro asks if it is true that he had been practicing swordplay while practice mining, which Mushashi affirms. An officer says that the possession of a Kitetsuki is a capital crime and that they will die, and has another but bigger Oni attack them, where then Kojiro realizes that his father was right about Oni. Musashi "sheaths" his sword which he regained from Kojiro in the ground as a technique from the Kanemaki Isshin School of Swordplay, which is thought to be a moronic move by an officer, and performs the Thousand Spiral-Rending Decapitation Blade which slices the Oni in half to everyone's amazement.
Musashi tells Kojiro that he doesn't want to be a Bushi by himself and would like themselves to be partners together. The "top-brass" Oni, the Kinshin, comes out and Kojiro and Musashi say they want to kick some Oni butts as they have always dreamed.