Genji Togashi, Shōei, visits his brother's grave and tells him that he, too, will be enrolling in Otokojuku, a school famed for forging young men through grit and discipline. Meanwhile, the young Yakuza successor Hidemaro Gokukouji, [Hiroyuki Onoue], parties with troublemakers from Kanto Gogakuren and ends up getting into trouble, only to be saved by Momotaro Tsurugi, [Tak Sakaguchi]. Hidemaro’s mother, [Tomoko Nakajima], has already decided to send him to Otokojuku to become a true man, setting the stage for the harsh journey ahead.
At the enrollment ceremony, Hidemaro is roughened by the head instructor Oni-Hige, [Shun Sugata], for not getting in line properly. The principal, Heihachi Edajima, [Akaji Maro], is introduced and his stern presence signals the brutal code of the school. During orientation, the 1st years Tazawa, [Taketori no Yamaguchi], and Matsuo, [Yoshiaki Yoza], are forced by Tange, [Masaki Miura], and the 2nd years to perform ridiculous tasks and manly, over-the-top displays. Togashi and Toramaru stand up to Tange, sparking a brawl in which Momo naturally gets drawn in. The clash is interrupted by the 2nd year leader Gouji Akashi, [Tetsushi Tanaka], who challenges Momo to a sword fight, only to be cut off by Oni-Hige before a victor can be declared.
During a uniform inspection after a scout for a pair of underwear, drill instructor Iron Helmet, [Kentarō Shimazu], declares that real men wear fundoshi and that the owner of the briefs will be punished. Togashi figures out the underwear belongs to Hidemaro and confesses his own preference for briefs, a bold move that lands him in a tub of hot oil with a candle on a leaf while wearing nothing but a fundoshi. He endures as the oil heats, a symbolic demonstration of perseverance meant to drive home a message to Hidemaro. Hidemaro, unable to bear it, slips away in the night, but after words of encouragement from Shioya, [Tatsuo Yamada], the last surviving member of the Gokukouji clan, he returns just as Momo faces scolding from Oni-Hige for letting him escape. The two are then imprisoned in twin cells, one of which has a 500-kilogram collapsible ceiling that Momo must hold up to keep Hidemaro from being crushed.
The Otokojuku campus comes under invasion by Kanto Gogakuren, a delinquent gang led by Omito Date, [Hideo Sakaki], a former Otokojuku student expelled for killing an instructor during a prior punishment. Date and his two right-hand men, Hien [Go Ayano] and Gekko [Shuya Yoshimoto], overwhelm most of the students. Akashi is nearly killed before Momo steps in to save him, and Edajima interrupts the confrontation to propose settling things through the legendary 3 Great Astonishing Assaults tournament. Momo, Togashi, and Toramaru are chosen to represent Otokojuku, a moment that marks both a personal and collective test for the trio.
Togashi returns to his brother’s grave, where Oni-Hige appears again and passes on his brother’s dosu knife, a symbol of ancestral resolve and manhood. As Momo trains his sword, Toramaru wrestles a bear, and Matsuo hones his ōendan, Hidemaro and Tazawa wonder what more they can do to contribute to Otokojuku’s rugged, unyielding tradition, each pushing themselves toward a future that demands strength, loyalty, and an unbreakable will.
