Knuckleduster arrives as an unexpected visitor in Koichi Haimawari's home. Knuckleduster doesn't mind if Koichi calls him master, but Koichi has no intention of becoming his disciple. Getting down to business, Knuckleduster explains that the drug he is searching for is called "Trigger". Trigger is a drug that significantly boost the user's Quirk while simultaneously weakening their sense of reasoning; Trigger is therefore a drug that can turn normal citizens into villains. Elsewhere, the three thugs are offered "splendid medicine" by the salesman. The medicine they are offered is none other than Trigger. The two thugs inject Trigger which boosts their Quirks respectively. Having seeing the enhancements, the salesman insists that the thug leader injects Trigger. Back at Koichi's home, Knuckleduster refuses to impromptu villains to roam about and won't allow that to happen. However, Koichi believes that it is better to let the Police Force or the Pro Heroes handle this situation. Knuckleduster explains that the Police Force and the Heroes are one step behind because it is impossible to distinguish people who have used Trigger, which is why it is up to them to stop these "Trigger Junkies". Knuckleduster states that their mission is to confront any thugs and make them stick out their tongues because Trigger causes tongues to turn into a hideous black color. Knuckleduster explains that they will beat them, make them talk about the dealer who gave them Trigger and cut off the circulation of Trigger. Koichi thinks that Knuckleduster isn't being serious.
However, Koichi becomes surprised when Knuckleduster does indeed start preparing to beat some thugs. Koichi manages to stop them by asking the thugs to stick out their tongues; they do so and their tongues are normal. After the thugs leave, Pop Step arrives and wonders what Knuckleduster and Koichi are up to. Pop Step is told that Knuckleduster is searching for people who have taken Trigger and their dealers in order to stop the circulation of Trigger. Pop Step thinks that they should use their heads instead of violence. Pop Step suggests that they should search on the internet for "sudden villain appearances" which will allow them to pinpoint the location of the Trigger Junkies making it easier and more productive to find potential Trigger Junkies. Knuckleduster spots a salesman and starts with him, making the salesman drop his suitcase which has Hero figures. Knuckleduster finds the salesman to be suspicious, but Pop Step and Koichi try to stop the old man to no avail.
Before Knuckleduster can punch the salesman to get answers, his arm is wrapped by cloth. The person stopping Knuckleduster with the cloth is Shouta Aizawa, best known as Eraserhead. Shouta believes that Knuckleduster is a villain which causes Knuckleduster and Shouta to start fighting, much to Koichi and Pop Step's shock. Knuckleduster doesn't take any serious damage from Shouta's kick and prepares to punch back. Having heard that Knuckleduster is rumored to process a stamina enhancing Quirk, Shouta activates his Eraser Quirk. As Knuckleduster and Shouta continue fighting, Knuckleduster apologizes to the salesman and asks him to declare his and Pop Step's innocence to Shouta. The salesman leaves in a hurry. Pop Step realizes that she has seen the salesman before; he appears in the pictures of villain disturbances. Pop Step tells Koichi to run after the salesman because he is suspicious.
Knuckleduster and Shouta fight on even terms with Knuckleduster managing to hold his own against the Erasing Hero despite the fact that Shouta has activated his Erasure on Knuckleduster. After being pushed back by Knuckleduster, Shouta is surprised that the old man has not lost any strength which causes Shouta to realize the truth about Knuckleduster.
Meanwhile, Koichi chases after the salesman but are stopped by the three thugs from yesterday. However, this time the three thugs have had their Quirks enhanced by Trigger; Pop Step continues chasing after the salesman while Koichi distracts them. The thug leader prepares to take his revenge on Koichi. Shouta stops his fight with Knuckleduster as he has realized that Knuckleduster isn't a villain after all. Knuckleduster takes Shouta's apology and they go their separate ways. Shouta thinks that Knuckleduster is a suspicious character but cannot charge him with any crimes. Shouta believes that because his Erasure had no effect, Knuckleduster is in fact Quirkless. Just in case, Shouta files a report on Knuckleduster to the Police Force.
As Shouta is busy filing a report, he hears screams; Koichi is using his Sliding Quirk to escape from the two Trigger-enhanced thugs. Koichi manages to dodge the thug leader's attack but is confronted by the other Trigger-enhanced thug. Knuckleduster appears and smashes a brick onto the Trigger-enhanced thug. The thug leader attempts to attack Koichi, but Shouta uses his cloth to stop the thug leader in his tracks. Seeing that Erasing Hero is handling the thugs, Knuckleduster and Koichi decide to go and find Pop Step.
Pop Step is chasing after the salesman but the remaining Trigger-enhanced thug are right on her tail. The thug kicks Pop Step causing her to fall on the salesman which in turn makes him drop his suitcase again. The salesman takes the figures and undoes their heads; the figures are actually Triggers in disguise. The salesman injects himself with Trigger which causes the salesman to become a colossal giant with four arms.
The colossal salesman giant grabs Pop Step and the thug with his hands, flaying them about. Knuckleduster leg drops the colossal salesman in the face, but the colossal salesman retaliates and smacks Knuckleduster into the ground. However, the colossal salesman's hold of Pop Step weakens as a result of Knuckledusters actions and she begins falling. Koichi goes to check on Knuckleduster, but sees Pop Step is falling to the ground and isn't able to stop due to no footholds. Koichi uses his Sliding and slides on a building. Koichi climbs the building and jumps off to towards Pop Step. The reason Koichi decided to help her is because even though it is far too late to become a Hero, he still wants to soar through the skies like her. Seeing Koichi, Pop Step jumps off him and activates her Bound Quirk which enables her to become afloat.
As Pop Step flies on to a building for safety, Koichi begins falling to the ground. The colossal salesman takes notice of Koichi and prepares to palm him. With quick thinking, Koichi decides to slam into the colossal salesman's palm and activates his Sliding. Koichi uses his Sliding to shift the trajectory of his fall and he lands onto a pile of garbage, safe and sound. However, the colossal salesman launches a punch at Koichi.
Suddenly, the colossal salesman starts shrinking and returns back to normal. Shouta activated his Erasure which forced the salesman's Gigantification Quirk to become disabled. Shouta wraps his cloth around the salesman. The Police Force arrive and arrest the salesman and the three thugs. Shouta thanks Knuckleduster for his assistance but finds his actions to be dangerous. Knuckleduster doesn't mind helping since he is a "virtuous citizen" and thus it is natural to cooperate with Heroes. Knuckleduster believes that Heroes need people like him so that the incident with the colossal salesman and the three thugs won't happen again since he is free to deliver justice without restrictions. Shouta finds it ridiculous that Knuckleduster believes his unlawful justice are crimes of conscience and finds him to be a troublesome old man. Knuckleduster is happy to be called a troublesome old man since he gets that all the time.
In a back alley, Pop Step is relieved that Koichi is alright. Koichi thinks that he isn't cut out to be a Hero. Pop Step tells Koichi to stop belittling himself and turns her back on him. Pop Step was going to thank him, but since he is annoying her she doesn't, while she blushes. Knuckleduster appears and is pleased that Koichi has learned how to land properly like him. Koichi still doesn't want to be Knuckleduster's discipline since he never wanted to in the first place and only called him "master" when he fell to the ground as a spur of the moment. Knuckleduster knows that Koichi's spontaneous words and actions are actually his true feelings; that he still aspires to be a Hero. Knuckleduster encourages Koichi to chase after his aspiration to be a Hero.
Thus, Koichi's life as a Vigilante began from a lonely back alley and he would eventually go on to reach for greater heights.