The plot begins when Takiya Genji (Shun Oguri), the son of a Yakuza boss named Takiya Hideo, enters Suzuran All-Boys High School as a transferred senior with the goal of conquering it. During the freshman orientation lecture, Yakuza arrive at the school seeking third-year senior Tamao Serizawa to avenge an injury. To show his worth, Genji fights and defeats the Yakuza, who mistake him for Tamao. One of the Yakuza, Ken Katagiri is impressed by Genji's skills. Meanwhile, Tamao greets his best friend Tokio Tatsukawa, who is discharged from a hospital, claiming that he is in good health. Tamao quietly dismisses Tokio claim and takes his bike to ensure he won't get harmed, slightly insulted that Tokio would lie to him. Tamao drives back to the school to witness Genji brutally finish off the last of the yakuza thugs. The two briefly take in the looming threat of each other's presence before Tamao is arrested for not having a driver's license for a bike.
After the intro, Genji meets Ruka in a nightclub, where she sings as an R&B singer. Genji declares his ambition of conquering Suzuran to his father, who failed in the task. They agree that if Genji conquers Suzuran, he may inherit his father's Yakuza family. The next day, Genji meets Serizawa and Tokio, who was a childhood friend of Genji's. Tokio manages to halt a potential fight in between the two and tells Genji to take down Rindaman before approaching Serizawa. Rindaman, who is giant-sized and a legendary figure in the school, ignores Genji, who instead finds Ken Katagiri. Ken attacks Genji for getting his gang arrested but Genji takes him down in one punch. Ken surrenders and takes Genji to a quiet bar to chat about Suzuran.
Ken tells Genji that he was in Suzuran as well, and came close to ruling it, but failed and ended up being a dropout. He proposes teaching Genji about leadership and influencing others to join his army, in order to correct Genji's lack of strategy in his ambition. Genji follows Ken's advice that he writes in a small notebook and manages to easily conquer Chuta Tamura's class, admitting him into his army. The Mikami Brothers of Class B join Serizawa's army, who then approach Class C's leader, Takashi Makise, who maintains a neutral position, claiming that even if Serizawa beats him in a fight, his men will never follow an enemy leader.
Genji decides to acquire Makise by exploiting his weakness for sex. Ken arranges a group date, but due to Makise's obvious awkwardness around women, the date flops miserably. Makise joins Genji anyway, based solely on the friendship formed on the day. Meanwhile, a gang known as the Freshmen Trio, have managed to take control of their grade level. They are soon provoked by Izaki, a notorious and calculating senior. Izaki proposes joining Genji, but ambushes him instead, to test his strength. Genji fights valiantly, and Izaki is impressed, despite thoroughly defeating Genji . He joins forces with Genji, forming the Genji's Perfect Seiha Army.
Serizawa is alarmed by the rapid rise of Genji, but chooses not to take action. A member of his army, Tokaji takes covert action against the perceived threat by ambushing Izaki and hanging him from a rope, bloodied. This provokes Genji, unused to caring for his fellow men and claims that, "This really hurts." Makise stops him from making a rash attack on Serizawa, their army still too small to hold a major fight. Later, Serizawa and Tokio meet Genji in a pub, but Tokio suffers a seizure and is rushed to a hospital, where he learns that he has a large cerebral aneurysm, and that the surgery required has a 30% success rate. Tokio chooses to go ahead with the operation, confident of his survival.
Angered by the provocations, Genji attacks Serizawa's classes even as Makise tries to stop him. Chuta is also ambushed by Tokaji, and as Genji tries to leave to attack Serizawa, Makise finally decides to leave the GPS Army, stating that he cannot follow a leader without control. Ken tries to encourage Genji, and learns that Genji 's father is Hideo Takiya, a former associate and head of the Yakuza family feuding with Ken's.
Tokaji covertly tries to arranges a deal with Hideto Bandou, a second year and leader of a biker gang, The Front of Armament, to kidnap Ruka to further provoke Genji and throw him off his focus. Meanwhile, Genji challenges Makise to a fight, with a condition that if Genji wins, Makise joins him again. Makise forgives Genji with their familiar, "Take all," gesture the two shared on the night they became friends. Genji holds back his tears of gratitude and playfully flips off Makise, the bond sealed again. Ken's boss, Joji Yazaki asks Ken to kill Genji, disregarding the fact that killing the heir to a Yakuza family would entail an all-out war. Izaki returns from the hospital and is given an awkward but warm welcome back by the Genji and his gang. Ken's order to kill Genji bears too much on his conscience and he informs Genji's father Takiya Hideo of Yazaki's plan to kill his son. Ken decides that he will not do as he was asked, and suffer Yakuza punishment for disobedience - death. Ruka calls Genji, telling him that she is abducted and that her abductors wore jackets with skulls on the backs. Realizing that they are the Armoured Front, the GPS Army gathers and makes their advancement to the Narai Warehouse, The Front's headquarters.
A fight ensues, and Bandou is impressed by Genji 's strength. He calls an end to fight to ask why Genji has chosen to oppose The Armoured Front. After Genji states that Ruka was abducted by men wearing jackets bearing skulls Bandou reveals that he has sealed the skulls until his group gains control of The Armoured Front and that none of his men bear the skulls on their backs. Further respecting Genji for his willingness to lose his ears in place of Makise as payment for the unprovoked attack, Bandou stops the knife hand and tells Genji that Tokaji might be behind the whole farce and where he can find him. Finding Ruka there, Ayman is finally bent for war against Serizawa. Serizawa, who was not aware of Tokaji's actions, punishes him and agrees to Genji 's proposal for an all-out battle on the school grounds. They decide to fight at 5 PM, at the same time when Tokio will enter operation, the idea being that Serizawa will fight alongside Tokio. The two share a cold stare down and leave. The next day, the two armies meet across the rain soaked school ground, Serizawa' army having 100 strong, GPS, 70. The two armies shout their battle cries and charge at each other.
As the battle enters full swing, the Freshmen Trio comment on the battle's balance. The battle goes in Serizawa's favour, even though the GPS army fights strongly. Bandou's section of The Armoured Front joins the battle midway on the GPS' side, evening the odds. Towards the end, only Serizawa and Genji remain standing, and fight. Meanwhile, Ken's boss takes him to a dock to be executed for his disobedience. He gives Ken his coat, stating that Ken had always wanted to wear it (aspired to becoming a Yakuza leader). He then shoots Ken, who cries out to Genji before sinking.
The battle drags into the evening as Genji and Serizawa continue to pound on each other, both badly damaged and exhausted. Genji gains the upper hand and wins the fight against Serizawa, who receives a call informing him that Tokio's operation was a success. Ken wakes up in the sea suddenly and swims up to the surface, realising that the coat he wore was bulletproof. Having defeated Serizawa, Genji finally meets Rindaman, stating that he has to be defeated if Genji is to be titled the king of Suzuran. Rindaman replies that Suzuran can never be conquered, as there is always someone left to defeat. The film ends as Genji attacks Rindaman.