Chokkaku is the eldest son of Ishigaki Kotayū, a low-ranking samurai of the Chōshū domain. His father gave him his name with a charge: to live straight, with nothing to be ashamed of as a samurai — turning even a corner at a right angle. True to that name, the boy has grown up perfectly straight. This spring he passes the entrance examination for the domain's most prestigious academy, Hagi Meirinkan, and begins his days in pursuit of bushido. But at the entrance ceremony itself, Chokkaku already finds himself in trouble with the upperclassmen.
Gō Shintarō, beaten by Chokkaku in a sparring match, vows to redeem himself as a samurai and openly challenges him at the school's martial arts tournament. Yet their homeroom teacher, Hosokari-sensei, does not pick Chokkaku as the representative of the Kiku class. Chokkaku settles for cheering from the sidelines — until his classmates' dismal showing wears out his patience, and he draws his wooden sword in defiance of the teachers' shouts. His showdown with Gō is about to begin.
After the tournament, Chokkaku and Gō become fast friends. But word of their daily brawls reaches Chokkaku's father. Angry and grieved, Kotayū declares that if Chokkaku ever draws his wooden sword in a fight again, he will be made to commit seppuku. Meanwhile, Tsukigata and his gang lord it over the academy, using brute force to do as they please. Unable to stand their tyranny, Gō urges Chokkaku to teach them a lesson — but Chokkaku has a promise to keep.
Keeping his promise, Chokkaku endures a savage beating alongside Gō. Disappointed that he will not fight back, his friends drift away. Tsukigata grows ever more arrogant, ordering every student in the Kiku class to strike Chokkaku and punishing a boy who refuses. Chokkaku cannot stand by and watch. Deliberately breaking his promise to his father, he settles the score with Tsukigata and his gang using his signature move, the Chokkaku Slash.
Hearing of the brawl with Tsukigata, Kotayū prepares to commit seppuku — but the castle steward, Tachibana Kaoru, stops him. Tachibana thinks highly of Chokkaku's cheerful, upright spirit. The ban on wooden swords is lifted, and Chokkaku is free again. It is at this moment that Hitomi Rintarō transfers in. Bright and academically gifted, Hitomi quickly becomes the most popular boy in the class. Only Gō, having caught wind of rumors about him, grows wary.
Hitomi Rintarō, the toast of the academy, asks Chokkaku to train him in swordsmanship. Hitomi's form is clumsy — yet Chokkaku senses one flash of sharp killing intent and beats him down, earning the resentment of his classmates. Then one day Tachibana summons Chokkaku to the castle. He tells the boy to become the heart of the academy and orders him to stay and think it over. But tonight is the night Tsukigata has challenged him to a duel.
Chokkaku skips the duel with Tsukigata and spends the night turning Tachibana's words over. All he works out is that some force appears to be at work, shaking Hagi Meirinkan to its foundations. Meanwhile, Tsukigata's gang resumes its bullying, and the Kiku class — with Hitomi at its center — stands against them. Chokkaku returns just as violence is about to erupt, defeats Tsukigata once more, and wins back the students' support. At last Hitomi reveals what he is really after and challenges Chokkaku to a one-on-one match.
"I want you to make Hagi Meirinkan an academy of both letters and martial arts," Tachibana tells Chokkaku. Taking the words to heart, Chokkaku sets up a training ground on campus and puts out a call for members — but only Gō and two unreliable students turn up. Just as he is sinking into despair, an invitation to a martial arts tournament arrives from the newly founded Daimon Academy. Convinced that a strong showing there will double their numbers, Chokkaku and Gō sign up at once.
The tournament finally begins — but it was arranged by Daimon Academy to drag Hagi Meirinkan's name through the mud. In the tense air, Hagi Meirinkan struggles; even Gō, superb with a sword, can barely get past three opponents. With everything on the line, Chokkaku racks up win after win with the Chokkaku Slash and draws roars from the crowd. Last of all stands Daimon Academy's finest fighter — the very center of that school, and a master of the martial arts.
Chokkaku's performance at the Daimon Academy tournament becomes the talk of Hagi Meirinkan. Welcomed back with cheers, he founds a kendo club to root the ideal of letters and martial arts in the school. Recruitment is hard going at first — and the ones who lend a hand are Tsukigata and his gang, Chokkaku's foes since the day he enrolled. Just as the castle steward Tachibana intended, Hagi Meirinkan begins to come together around Chokkaku.
With help from Tsukigata and Gō, the Hagi Meirinkan kendo club is steadily gaining members. Chokkaku's training is a little over the top, but his friends are visibly improving, and he begins to feel that turning the academy into one of both letters and martial arts is within reach. Then one day a transfer student arrives: a boy who has been studying Western learning in Nagasaki. His name is Hōjō Terumasa, grandson of the former castle steward, Hōjō Terumori.
Word of Chokkaku's exploits at the Daimon Academy tournament spreads beyond the school. Hearing of it, the all-female Shigetsu Kendo Club sends him a written challenge. Tsukigata and the others sneer and refuse to take it seriously — yet led by their captain Kigisugi Jun, every one of them is highly skilled. Jun challenges Chokkaku to single combat, and her attacks launched from horseback leave him purely on the defensive. How will the fight end!?
Jun's sharp swordsmanship drives Chokkaku back to the edge of a sheer cliff. Just when all seems lost, he seizes a single instant and turns the fight around. Jun is impressed — and she has also noticed that, throughout the exchange, Chokkaku kept positioning himself so that she would not fall. Watching the two of them, Terumasa loses patience and challenges Chokkaku to single combat…
When Kotayū learns that Chokkaku injured Terumasa in their duel, he works himself into a frenzy over how to apologize. No reprimand comes, however, and the relieved Ishigaki father and son enjoy a bath in the garden. It is then that Jun visits the Ishigaki house. She speaks to Chokkaku of how vital it is that bushido be passed on correctly at Hagi Meirinkan, laments that she was not born a man, and promises him her full support.
Encouraged by Jun, Chokkaku installs at Hagi Meirinkan the same contraptions his father used to toughen him up in the garden at home. His friends are bewildered at first, but Gō and Tsukigata soon learn to dodge the traps' attacks. The training pays off handsomely — except that it gets in the way of studying. As Chokkaku frets, Jun proposes a plan: build a proper dōjō so the students can throw themselves wholeheartedly into martial practice.
Everyone agrees to the dōjō. Jun will pay half the cost, and the kendo club members will make up the rest between them. Then Chokkaku realizes something: unlike the other students at Hagi Meirinkan, the Ishigaki family are low-ranking samurai. They are not well off, and he cannot possibly raise the matter of money with his father. He tries to earn the funds himself with a stall at the festival, but nothing sells. Just as he is at his wit's end, encouragement comes from Katsuragawa-sensei — the teacher who recommended him to Hagi Meirinkan.
Heartened by Katsuragawa-sensei's words, Chokkaku catches wind of a sumo tournament — with a prize of five ryō! To raise money for the dōjō he enters with his father, and his nimble footwork carries him past four opponents. Then the fifth steps up: a mountain of a man who seems to tower out of sight. With Ayano and Jun cheering him on, Chokkaku uses speed to run the giant ragged, but at last the man catches hold of him…
With the dōjō funds finally in sight, Chokkaku can breathe easy — until the castle steward Tachibana comes calling. Holding Terumasa's guitar, Tachibana bows his head and asks Chokkaku to become the boy's friend. Chokkaku accepts readily, visits the Hōjō mansion, and takes Terumasa out into the town. Guided by Jun, they arrive at the newly built dōjō, where Chokkaku throws himself into practice with his friends. In a corner of the hall, Terumasa is left standing all alone.
He was meant to become Terumasa's friend, yet he left him feeling lonely. Chastened, Chokkaku invites Terumasa to his home. The whole family is delighted, heating the bath and preparing a meal, spared no effort in welcoming him. Terumasa keeps up his usual sour remarks, but with Chokkaku's mother he turns honest and open. Chokkaku is satisfied that they have grown close — but that night, gazing up at the moon, Terumasa wears a lonely look.
On Hōjō Terumori's orders, Terumasa's study companions are to be chosen from Hagi Meirinkan. Named are the well-born Yukisa Takuma, the academically brilliant Gō, and Chokkaku. Chokkaku moves into the Hōjō mansion, where for some reason Yukisa treats him with open severity. Chokkaku pays it no mind and struggles to become a good friend to Terumasa — while Terumasa, for his part, tries to make trouble for him, acting spoiled and setting nighttime pranks…
