Whenever he isn't getting pummeled in the karate club after school, high schooler Kenichi Shirahama can be found with his head buried in one of the many self-help books he carries with him every day. One day, on the way to school, Kenichi accidentally frightens the new student in his class, leading her to throw him down onto the ground. After apologizing for her violent reflexes, she introduces herself as Miu Furinji and asks Kenichi to be her friend. Later that day, after karate club practice, Kenichi is given an ultimatum by one of the other club members: they will fight next week and the loser has to quit the club permanently. While lamenting his imminent defeat in the upcoming fight, Kenichi witnesses Miu fend off three thugs easily. Fed up with being perpetually bullied by those more powerful than him, he asks Miu for tips on how to become a better fighter. She directs Kenichi to her grandfather's dojo, Ryouzanpaku, where several skilled—albeit strange—masters of extreme martial arts have gathered to hone their abilities. Miu promises that Kenichi will become significantly stronger, but only if he manages to "stay alive." With those ominous words looming in his mind and the appearance of a criminal martial arts gang known as "Ragnarok" threatening the safety of those around him, Kenichi takes the first step on his journey to become a world-class fighter and a defender of the defenseless. "Weak Legs" Kenichi Shirahama would rather spend his time reading self improvement books than fighting. However, when he finally works up the courage to become strong and join his school's karate club, he is coerced into fighting a bullying upperclassman who is intent on getting him kicked out of the club. He is about to give it all up until he falls for his mysterious new classmate, Miu Furinji. In order to face this challenge, he undergoes rigorous training at the dojo she lives at, Ryouzanpaku. Some initial training by the masters there allow him to defeat his upperclassman, however his fighting prowess brings him to the attention of the powerful gang of delinquents, Ragnarok. Wishing to protect the things he loves and determined to have the strength to face the increasing adversity, he must learn various martial arts from the dojo's resident masters, taking Karate, Muay Thai, Ju Jitsu and Chinese Martial Arts and combining them to create his own fighting style!