Knuckleduster arrives as an unexpected visitor in Koichi Haimawari's home. Knuckleduster does not 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. Raputo Tokage and Moyuru Touchi inject Trigger which boosts their Quirks respectively. Having seen the enhancements, the salesman insists that Souga Kugizaki inject Trigger. Back at Koichi's home, Knuckleduster refuses to let impromptu villains to roam about and will not 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 is not 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. Koichi tells her that Knuckleduster is searching for people who have taken Trigger and their dealers in order to stop the circulation of Trigger. She thinks that they should use their heads instead of violence, and 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 Shota Aizawa, best known as Eraserhead. Shota believes that Knuckleduster is a villain which causes Knuckleduster and Shota to start fighting, much to Koichi and Pop☆Step's shock. Knuckleduster does not take any serious damage from Shota's kick and prepares to punch back. Having heard that Knuckleduster is rumored to possess a stamina enhancing Quirk, Shota activates his Erasure. As Knuckleduster and Shota continue fighting, Koichi apologizes to the salesman and asks him to declare his and Pop☆Step's innocence to Shota. The salesman leaves in a hurry. Pop☆Step realizes that she has seen the salesman before; he appears in the pictures of villain disturbances. She tells Koichi to run after the salesman because he is suspicious.
Knuckleduster and Shota fight on even terms with Knuckleduster managing to hold his own against the Erasing Hero despite the fact that Shota has activated his Quirk on Knuckleduster. After being pushed back by Knuckleduster, Shota is surprised that the old man has not lost any strength which causes Shota to realize the truth about Knuckleduster.
Meanwhile, Koichi chases after the salesman but is stopped by Souga, Raputo, and Moyuru. Souga prepares to take his revenge on Koichi and injects himself with Trigger. Shota stops his fight with Knuckleduster as he has realized that Knuckleduster is not a villain after all. Knuckleduster takes Shota's apology and they go their separate ways. Shota thinks that Knuckleduster is a suspicious character but cannot charge him with any crimes. Shota believes that because his Erasure had no effect, Knuckleduster is in fact Quirkless. Just in case, Shota files a report on Knuckleduster to the Police Force.
As Shota is busy filing a report, he hears screams; Koichi is using Slide and Glide to escape from the Trigger-enhanced thugs. Koichi manages to dodge Souga's attack but is confronted by Moyuru. Knuckleduster appears and smashes a brick onto Moyuru's head. Souga attempts to attack Koichi, but Shota uses his Capturing Weapon to stop the thug 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 Raputo is right on her tail and kicks her, causing her to fall on the salesman, who drops his suitcase again. The salesman picks up the All Might doll, pops off its head, and uses it to inject himself with Trigger. He then grows into a colossal giant with four arms.
The colossal salesman giant grabs Pop☆Step and Raputo with his hands, swinging 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 is not able to stop due to no footholds. Koichi slides up the side of a building and jumps off 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. Instead of being caught by Koichi, Pop☆Step uses him as a platform for her Leap Quirk.
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 Quirk, shifting the trajectory of his fall and landing 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. Shota activated his Erasure which forced the salesman's Gigantification Quirk to become disabled. Shota wraps his cloth around the salesman. The Police Force arrive and arrest the salesman and the three thugs. Shota thanks Knuckleduster for his assistance but finds his actions to be dangerous. Knuckleduster does not 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 will not 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, but he thinks that he is not cut out to be a Hero. She tells him to stop belittling himself and turns her back on him. She was going to thank him, but since he is annoying her she does not, while she blushes. Knuckleduster appears and is pleased that Koichi has learned how to land properly like him. Koichi still does not want to be Knuckleduster's disciple 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 begin from a lonely back alley and he would eventually go on to reach for greater heights.
Characters in Order of Appearance
- Koichi Haimawari
- Raputo Tokage
- Moyuru Touchi
- Mario Kugutsu
- Souga Kugizaki
- Kazuho Haneyama
- Shota Aizawa