Eccentric and blunt, the walrus Hiroshi Odokawa lives a relatively normal life. He drives a taxi for a living, and there he meets several unique individuals: the jobless Taichi Kabasawa who is dead-set on going viral, the mysterious nurse Miho Shirakawa, the struggling comedic duo "Homo Sapiens," and Dobu, a well-known delinquent. But Odokawa's simple way of life is about to be turned upside down. The case of a missing girl the police have been tracking leads back to him, and now both the yakuza and a duo of corrupt cops are on his tail.

October 4 marks the fateful day when a high school girl vanished, triggering an unpredictable series of events. In order to unravel the mystery behind the girl's disappearance, all of the key witnesses connected to the case recount the events leading up to and following the crime. As the pieces to the story come together, they all seem to be connected to one individual—taxi driver Hiroshi Odokawa.

Freed from the rule of samurai 150 years ago, the people of Hinomoto live peaceful lives and revere their liberators—demons whose true forms remain elusive—as gods. Fated to mine rocks for the demons to feast on, Musashi knows the grim truth: humanity is enslaved by these otherworldly beings and the samurai are the last bastion fighting for freedom. Hiding his true opinion from his peers and growing distant from his childhood friend Kojirou Kanemaki, Musashi lives in angst until the day of his graduation. When the fresh graduates arrive at the mine, they are horrified to see the inhumane treatment of miners and the uncanny physical appearance of their overseers. However, Musashi manages to endure thanks to Kojirou's help. As the demon leader wreaks havoc on the quarry, Musashi is saved once more from certain death by the Takeda samurai clan. Recovering from humiliation and yearning for adventure, Musashi embarks on a journey to become a samurai and form his own clan!

After Musashi rescues Michiru Saruwatari from a demon, his friend Kojirou Kanemaki discovers that his father was a member of the Uesugi clan. Determined to learn more about his own past, Kojirou—alongside his Kanemaki clansmen—decides to participate in Uesugi's punitive expedition against the demon lord of Awaji Island: Yamata no Orochi, an eight-headed dragon that threatens to engulf the entire region. However, upon joining Uesugi's military alliance, Musashi faces the harsh reality of fighting against powerful demons. Humiliated by his lack of power and inability to work in groups, he has to learn how to cooperate with his newfound allies. Meanwhile, the Obsidian Eight—a group pursuing the Obsidian Goddess that dwells within Musashi's body—resumes its hunt for Musashi, and plans to thwart the ongoing island operation. As the situation grows chaotic, Musashi and his friends must foil the machinations of the various factions conspiring to seize the Obsidian Goddess.