Yaiba Kurogane's life goal is to become a true samurai and has been training hard in the jungle with his dad, Kenjuro, day in and day out. But training takes an unexpected and sudden turn with Yaiba and his dad, along with their tiger companion from the jungle, Kagetora, ending up back in Japan. Turns out Kenjuro has a friend who happens to be a member of the esteemed kendo family known as the Mines. The Mines agree to take in Yaiba, but Kenjuro's friend's daughter, Sayaka, is not as amenable. Yaiba's completely odd ways have Sayaka's patience wearing thin. It's when Yaiba visits Sayaka's school and meets the talented kendo practitioner Takeshi Onimaru, that tensions really rise. A fateful meeting is about to unfold.

Yaiba has come up with a plan and is willing to do almost anything to lure Onimaru into a duel, which Onimaru sees as nothing more than a sullying of his superior skill and reputation. It takes Yaiba's trusty vulture, Shonosuke, following him all the way from the jungle and trashing Onimaru's treasured pigeon house to flip a switch and rage. Yaiba manages to insult Onimaru further with his unconventional sword play and goading. When he vents his pent-up anger on an unsuspecting stone lantern in his backyard, Onimaru inadvertently uncovers a hidden stairway that descends to a secrete underground chamber. There a maken known as the Fujinken awaits.

This time it's Onimaru prodding Yaiba for a duel. Only something is seriously off, not least the disturbing new horns sprouting from his forehead. Still, undeterred, Yaiba takes on the strange new Onimaru, but is dejected by how quickly the Fujinken and its wielder can dominate him. To stand a chance against Onimaru, Yaiba's dad explains, he must get hold of another maken known as the Raijinken. To find it, Yaiba heads to Mt. Tengu to meet the famous swordsman Musashi Miyamoto, who also happens to be four hundred years old...

Yaiba may have gotten his hands on the Raijinken, but it's the Thunder God's power concealed within that has turned the tables on him. The blade's potency has practically swallowed Yaiba whole, leaving him in a state of frenzied rage! To take back control, Yaiba undergoes special training with the fabled swordsman Musashi. But time is running out. Onimaru, now a possessed oni, has unleashed the Eight Ogres. And having taken Japan's National Diet Building, Onimaru makes it clear he's after more than just political might. His demon eyes are steadied on Yaiba's Raijinken. To take his prize, he orders two of his henchmen - Mr. Frog and Mr. Snake - to do what they must.

Yaiba and team head to the infamous Ganryu Island to learn more about the enigmatic weapon known as the Monohoshizao, a maken once wielded by Musashi's one-and-only rival, Kojiro Sasaki. But one of Onimaru's minions, Mr. Slug, has other plans. Eager for an escape, Yaiba and his entourage pay a visit to the restorative hot spring Gero Onsen. Instead of finding relaxation, however, Yaiba and friends are ambushed by, of all people, a local hotel employee who claims they've just won an exclusive, all-paid stay at his luxurious hot spring resort. Yaiba couldn't be more excited, but someone - or something - is waiting...

With Mr. Sea Cucumber now part of the team, Yaiba and crew are on route once again to Ganryu Island, but not before making a pit stop in foodie heaven Nagoya. For a moment, as everyone chomps down on the local specialty miso kushi katsu, all seems right in the world. That is, until they hear his voice. Suddenly, Sayaka's kushi katsu disappears into thin air, a sign, as they find out, of a bigger problem: the self-proclaimed Mr. Bat, who has his sights on Yaiba's Raijinken. Turns out Mr. Bat is a formidable foe. Musashi may be quick to step in to help, but a swift bite to the neck changes everything.