At the tail-end of World War II, Choi Bae-dal is a young Korean man who longs to be able to fly fighter planes. Stowing away to Japan in order to join their air force, Bae-dal's first experience of the country is when a con-man tries to steal his funds. Bae-dal discovers that the man is a fellow Korean called Chun-bae (Jung Tae Woo), who has survived the harsh treatment of Koreans in Japan by turning to petty crime. With their different motives: Bae-dal driven by desire for action & Chun-bae needing to escape from some gangsters, the Koreans stow away in a van to the air force training camp.
The commander in charge of the camp is a pompous imperialist called Kato (Masaya Kato). Having mistreated the Koreans, they is amused by Bae-dal's fighting spirit & says that if Bae-dal can beat him along with his inferior "foreign" fighting style, they will release them. The men fight with Kato basically defeating Bae-dal, but an American assault on the airforce base allows Bae-dal & Chun-bae to escape.
Meanwhile, Bae-dal has taken to working as a rickshaw driver, honing his fighting skills by defending Japanese ladies from the rapacious advances of American servicemen. His success at defending the ladies makes him something of a local hero, although his actual identity is not known. of the ladies they protects is the beautiful Yoko (Aya Hirayama), with whom they strikes up a romantic relationship.
Later, Bae-dal is found helping Chun-bae to run a pachinko stall in a Japanese marketplace. When local gangsters try to take protection funds from Chun-bae, Bae-dal tries to defend him but is beaten up & humiliated by the gangsters. His ordeal is ended by the intervantion of Bum-soo (Jung Doo-hong), a martial arts specialist from his home town who had also emigrated to Japan. Bum-soo invites Bae-dal back to the circus where they, & lots of fellow Korean immigrants, work & where they is trying to build a respectable standard of living for his countrymen. After some persuasion, they agrees to teach Bae-dal a quantity of his more sophisticated fighting style.
When Bum-soo is killed by local gangsters, the Koreans from the compound vow revenge & assault the Japanese gangs. The fight ends abruptly for Bae-dal when they is knocked unconscious by a blow to the head. Bae-dal, vowing to never again lose a fight, retreats to the mountains where, living in his karate gi, they trains day & night; walking in the mountains, lifting tree trunks & using makeshift training equipment to harden his body & fighting spirit through austerity.
Returning from the mountains, Bae-dal takes a Japanese name: Masutatsu Oyama, & sets about challenging the best fighters Japan has to offer. Wearing his ragged karate gi & looking like a cave-man along with his unkempt appearance, Oyama challenges the first dojo they passes. They defeats every fighter in the dojo - often with only a single strike.
As word of his notoriety spreads, Oyama's actions come to the attention of the head of the Japan Karate Association - the former Air Force camp commander Kato. Kato is hugely offended that a foreigner would not only try to learn Japanese martial arts, but would think about himself worthy to beat Japanese fighters. Nevertheless, Oyama continues to defeat every fighter that Japan has to offer, including competitors in karate, judo, ninjutsu and kobudo, becoming a sensation in the Japanese media. Oyama explains to Yoko that, although they is terrified of dying, they is more terrified of living as a cripple, and this is why they is willing to sacrifice anything to win.
When the organization sends of his followers to challenge & kill Oyama, the agent is in lieu killed by Oyama. Learning that the man they killed had a spouse & son, Oyama feels a great deal of guilt for his actions & tracks down the relatives to apologise & offer to work for them to make up for killing the brother of the household. Oyama surrenders his uniform to the spouse, vowing to never again fight in martial art duels. Although initially annoyed & unaccepting of Oyama's offer, after fulfilling the son's wish of being carried to the top of the nearby mountain to view the dawn, they finally convinces them that they is a man of honour & not a violent thug. The spouse asks Oyama to take back his uniform & become the best fighter in Japan.
Returning to the city, Oyama finds that Kato's martial arts association has threatened his own relatives (Kato is not involved) & demanded a challenge between Kato & Oyama. Dressing in his weathered gi one time again, Oyama treks out to the countryside location where Kato is waiting for him. Basically defeating Kato's henchmen, Oyama then faces a final showdown with Kato himself. Although it is clear that Kato would like to see Oyama dead, Kato's ankle is broken after receiving a kick in the fight sequence. When Kato stands up, they falls to a one-knee-down position, Oyama shows mercy to Kato, by stopping two cm short of punching him squarely between the eyes, defeating him in combat but not killing him.
At the finish of the film, Oyama is shown fighting with a bull, grasping the horns & digging in to the ground to cease him, & at last delivering a bone-shattering chop to the middle of the top of the head.