In the past, Iwao Oguro was a Pro Hero known as O'Clock, While in China he was known as Ciupan Seikyun (Hyper Quadfist). His career as a Pro Hero came to end after his Quirk was stolen from him, his face was split open and he was presumed dead.
After an unknown amount of time passed following the incident, his relationship with his daughter, Tamao Oguro, becomes strained after she expresses her desire to become a musician with her friends, only for him to yell at her in protest and told her to stop hanging out with that group of people. This caused her to leave home.
Following Tamao's sudden departure, Iwao Oguro's wife is hospitalized and is currently unable to speak properly. In an effort to track down his missing daughter, the former Pro Hero becomes a Vigilante named Knuckleduster. During his first investigations, he discovers that the body of his daughter has been controlled by a parasitic bee known as Kuin Hachisuka, who works the Villain Factory, a criminal organization that traffic with Trigger and that seems to be responsible for the vegetative state of his wife.
Since then, Knuckleduster is looking for a way to rescue his daughter Tamao and put an end to Villain Factory's operations. To carry out his objective, he uses several aliases to hide his identity, as well as to have contacts with both the criminal world and the forces of law to obtain information and supplies.
The Formation of the Naruhata Vigilantes
Knuckleduster meets Koichi Haimawari and Kazuho Haneyama for the first time while performing vigilante tasks - looking for consumers of Trigger. He sees Koichi and Kazuho confronting Soga Kugizaki and his mates Raputo Tokage and Moyuru Tochi, and decides to intervene, defeating them with ease. After that, he examines their tongues to see if they’re discolored, but cannot find anything. However, impressed by Koichi's bravery, Knuckleduster decides to take him as his disciple to make him a hero, despite not having the license.
During the following weeks, Knuckleduster usually visits Koichi at home to train him. He also explains to Koichi and Kazuho the whole issue referred to Trigger, a drug, he elucidates, that gives a boost to an individual's Quirk while also weakening their sense of reason. He tells them that is characterized by leaving on its consumers’ tongues a characteristic black discoloration, which is what he was checking for on Soga and his friends.
During this period, they also carry out several acts of vigilantism, where they face several Impromptu Villains, as well as other minor criminals and oddities. In one of these missions, Knuckleduster tries to catch Mario Kugutsu, a Trigger dealer for the Villain Factory, who has recently supplied samples of Trigger to Soga and his friends. They then inject the drug into themselves, causing them to become Impromptu Villains and try to take revenge on Koichi for the beating they suffered in their last quarrel. He also meets and confronts Shota Aizawa, who later decides to help him to stop and arrest Mario, Soga, Raputo and Moyuru.
In one of these missions, he finds clues about Kuin.
Later, he faces Stendhal, another vigilante who, unlike him, does not hesitate to kill the criminals and anyone who dares stand in his way. Initially, Stendhal works with the likes of vigilantes such as Knuckleduster, The Crawler, and Pop☆Step. However, he becomes more and more disillusioned with hero society, to the point where he begins to work with a Kuin, who was supplying Trigger to several small-time criminals. One of the targeted criminals is Soga, who, after realizing the seriousness of his actions, decides to get away and remove his friends from everything related to Trigger, challenging Kuin in the process.
Stendhal intends to not only kill Soga, but also Koichi, who risked his life trying to save Soga. Stendhal uses his Bloodcurdle Quirk to paralyze them, but as he prepares to kill them, Knuckleduster intervenes. During the fight, he lays down a line of wire and tells the killer that he'll punch him if he steps over it. When Stendhal gets close, Knuckleduster steps over the line and lays him out flat, pointing out that he never said that he wouldn't cross the line himself. Stendhal's nose is completely smashed in and he’s forced to retreat despite only causing minor injuries on the Vigilante. However, his defeat is not a complete loss, as becomes enlightened after hearing Knuckleduster's comments on resolve.
After this, Knuckleduster continues with his investigation to find the whereabouts of Kuin, this time with the help of Soga, who is looking for clues for him.
Knuckleduster finally manages to find Kuin when she is about to release Teruo Unagisawa, a Next-Level Villain with the Eel Quirk, at the Narufest event. Koichi and Kazuho also participate along with other artists. Kuin thinks that by occupying the body of his daughter, Tamao, Knuckleduster will not dare to attack her, but she is wrong since Knuckleduster has no qualms about using his new weapon, a Taser Knuckle, against her.
Kuin tries to avoid facing directly Knuckleduster by using all kinds of tricks such as bees bombs, but Knuckleduster perseveres. During the fight, he receives a call from the hospital informing him of the worsening health condition of his wife. He responds that soon the whole family will be right over. Once cornered, Kuin does not hesitate to resort to more violent methods. After injecting herself with a sample collected from Teruo (who was defeated by Shota), she temporarily gains electrical powers that she channels through claws on her fingers, and manages to stop Knuckleduster’s heart in one hit.
Thinking that she has killed Knuckleduster, Kuin continues with her plan to crash the Narufest party, but Knuckleduster reappears and reveals that his Taser Knuckle also has a defibrillator function which allowed him to survive. Kuin attacks him again, but this time it is Knuckleduster who defeats her with an electric attack, using the Taser Knuckle. Knuckleduster manages to get her swarm to leave his daughter's body by stopping his daughter's heart. He then extracts the queen bee, Kuin, from her eye, and calls back the swarm with a pheromone and bomb combination device, which explodes, killing the queen bee and the swarm.
Knuckleduster manages to resuscitate his daughter and save her, taking her to the hospital where his wife is hospitalized, who unfortunately dies shortly after their arrival.
A Vigilante's Farewell
After defeating Kuin and saving his daughter Tamao, Knuckleduster continues as a vigilante along with Koichi and Kazuo, but for a time there seem to be no more incidents related to Trigger and Villains’ imprisonment, as they’re only defeating simple petty criminals. Koichi thinks that Trigger has stopped circulating in the streets, but Knuckleduster tells him that the people behind those incidents have not surrendered.
He visits his hospitalized daughter, who has been being taken care of by Soga. When Tamao asks for her mother, he simply replies that at that moment she can not see her. Shortly after, Knuckleduster disappears without saying goodbye to Koichi, although he leaves his characteristic Brass Knuckles and his mask in his house.
Return to Japan
After several weeks without knowing anything about him, it seems that Knuckleduster had ceased to be a vigilante after defeating Kuin and saving his daughter, but in reality, he traveled to Hong Kong, where he is still looking for clues about Trigger on his own.
After obtaining the necessary information from some dealers, he returns to Japan to visit his daughter in the hospital, and later, under his alias as Mr. Kuroiwa, he contacts detective Tanema to give him information about Japanese importers of Trigger. Apparently, Tanema knows Knuckleduster’s past, and he asks him if he knows anything about a villain who seems to have a similar type of quirk as O'Clock. Knuckleduster simply says does not know who he's talking about, but advises the detective that if they meet that villain again, do not hesitate to shoot his vitals.
Soon after, he observes Koichi from a distance, happy to see that he continues to fight crime, although he has not contacted him yet.
He shoots Number 6 and then confronts him.