The story of Shamo is split into three parts.

Part 1 the reformatory

Ryo Narushima (成嶋 亮 Narushima Ryō?), a talented high schooler who was about to enter Tokyo University, the most prestigious university in Japan, killed his parents before his successful life could begin. The perfect family and perfect life seemed to devour the young boy's restless soul. On a beautiful sunny afternoon when cicadas were singing, he stabbed his father and mother to death with a short knife repeatedly before he came to his senses.

The 16-year-old bookworm murderer was convicted and sent to a reformatory where he was gang raped by other boys. Kenji Kurokawa (黒川健児 Kurokawa Kenji?), a jailed man who nearly assassinated the Japanese Prime Minister decades ago, was sent to the reformatory to teach the youngsters karate every week. He discovered Ryo's talent and taught him self-defense. Ryo survived and thanks to a law that protects minors, he was released two years later, but not before beating several would-be attackers with such savagery that his previous tormentors became terrified of him. He also engaged Kurokawa in a sparring match as a final test of his skills. Ryo returns to society with the mindset of gaining strength so as to survive and never be a victim again.

Alone in a city full of crime, he tried to look for his lost sister but ends up mistaking a drug-abusing prostitute for her. Ryo started to fight for his living and used all imaginable dirty tricks to defeat his enemy. He also ambushed gangsters in dark alleys to perfect his fighting skill as well as working as a gigolo. He later enters into a Banryukai tournament and spends time training his reflexes by kidnapping a woman and forcing her to attempt to kill him, releasing her after he successfully sensed her killing intent. He wins the tournament by blackmailing his opponents anonymously and throwing them off their game with his knowledge of their dirtiest secrets (e.g. an opponent whose wife is openly unfaithful). He wins in his weight class despite objections by the crowd and his fellow contestants over his vicious fighting style, but his victory is legitimized by Mochizuki himself, who claims that Ryo's fighting is still superior no matter what the context.

Part 2 Sugawara

In this part, Ryo fights Naoto Sugawara (菅原 直人 Sugawara Naoto?) of the Banryukai (番竜会 Banryūkai?) twice. Kurokawa, a cast away from that elite dojo, helps Ryo to become stronger as a way to revenge Kensuke Mochizuki (望月 謙介 Mochizuki Kensuke?), his past foe and the current owner of the Banryukai. Banryukai is possibly modeled after the Kyokushin kaikan and Seidokaikan. The name Mochizuki is possibly modeled after Kazuyoshi Ishii, the founder of Seidokaikan.[citation needed]

Believing that he is the strongest, the "gamecock" starts to take on other good martial artists. A blood-thirsty TV producer notices that this street-fighting young man was the "Youth A" and pushes to have him join "Lethal Fight," a fictional combat arena modeled after Japan's K-1 tournament. Ryo, a dangerous man with an animal instinct who destroyed many good fighters in the ring, one day finds himself facing Thailand's best Muay Thai practitioner who was fighting to support his family. Before he is totally defeated, he tries to kick the Thai fighter's neck but the fighter ducks and is hit in the eye and blinded.

Ryo wants to take on Sugawara. However, Sugawara is much taller and heavier than Ryo was so his chance to fight him was very remote. As a way to provide incentive, Ryo raped Sugawara's supermodel girlfriend. Sugawara vowed to kill Ryo with his hands in the boxing ring to comfort his woman and agreed to a televised fight at the Tokyo Dome. It was the duel between darkness and brightness. Only this time, Ryo (亮), literally "brightness," stands for the dark side.

Ryo goes through a painful bodybuilding regime where he used steroids to increase his muscle mass and strength. Before the fight began, his left eye became bloody under the non-human torture. He did not care. Bad luck was on Ryo's side. His small body still was no comparison to Sugawara's. However, he rediscovered his long-forgotten left-handedness suppressed by his parents since he was a child minutes before the end of the last round. Then he wildly gave Sugawara countless heavy left punches before Sugawara used his broken right fist to punch him out of consciousness. He survives the lethal fight and was defeated only five seconds before the fight was over. Sugawara fails to kill him in front of the crowd.

Out of anger and frustration, Sugawara invites Ryo to another private fight three months later in an abandoned temple. Sugawara takes several darts and a long wooden stick. Ryo takes a pair of tonfa with him. After a long and brutal duel where Narushima is almost killed, Sugawara is hit in the back of the neck and was hospitalized.

Part 3 The Ballet Dancer

It tells a story that Toma Takahara (高原 東馬 Takahara Tōma), a successful male ballet dancer with a savior complex, mysteriously abandoned his dancing career to pursue martial arts. It is revealed that the reason is because, after witnessing one of Ryo's fights, he became obsessed with Ryo and desired to 'save' him like he has others before. The new books (20, 21) pick up with Ryo fighting in a club in Japan. Ryo discovers after one fight that his ability has dropped significantly (implied to be due to the experimental steroids he had taken before) and he decides to start training hard. Ryo enters a karate competition in a mask and after losing in the finals by disqualification, is spotted by Mochizuki. Mochizuki offers him money to enter a grappling tournament fighting against Toma. Ryo, desperate for cash accepts and begins training with the master of the man he lost to in the finals (who has since become a hikkikomori). Before the tournament, he visits the comatose Sugawara in the hospital and after Ryo implores his rival to wake up, Sugawara hears Ryo's voice and comes out of his coma. Ryo is placed on Team Banryukai, made up of himself and the four Black Dogi (Banryukai masters wearing black gi) and Team Toma is made up of Toma and four masters of various styles ranging from well known to esoteric. The first four fights end with two no-contests, two wins by team Toma and one win for the Banryukai. While waiting for his turn to fight, Ryo was stabbed by Moemi Funato. However, he still plans to fight.

Ryo and Toma start fighting with Toma at a heavy advantage. However, before he can win a sudden thunderstorm floods the stage. adapting more quickly to this new field Ryo turns the tables and begins using mind games to overwhelm the more naive Toma. Having so far fought calmly Toma becomes enraged and afraid further playing into Ryo's hands. The damage from the stab wound was too much however and Toma manages to win, but he is mentally crippled by Ryo's nightmarish fighting style and the hallucinations he had during the fight.

During the aftermath of the fight Mochizuki is replaced as chairman and the Banryukai is split into two factions (True and New Banryukai) and officially breaks all ties with Lethal Fight. After a night of drinking Mochizuki is mugged and killed. Ryo finds out that Kurokawa and his ringside doctor have both died during the night. Ryo begins living with his sister and her caretaker.

I got this info from wikipedia