The Fat Gum Agency, is a large building with a rounded roof located between two others. The main facade of the building is decorated to mimics the appearance of Fat Gum himself. The upper part looks like the BMI Hero's face, with a black area representing his mask with two round windows simulating his eyes. Below this area there is a row of windows positioned so that reminds Fatgum's distinctive smile.
The bottom of the facade has the initials of Fat Gum alias, with an F and an inverted F on the sides and a G surrounding the entrance door. The words "Fat Gum" are posted just between the initials as well. On the left side of the building there is a decorative element that represents Fat Gum's arm holding a takoyaki ball.
Inside, the office is austere, with a table, a huge dark chair and a few furniture.
Fatgum is contacted by Detective Naomasa to assist in a police operation. The police's investigations had discovered that a criminal organization known as the Villain Factory is preparing to illegally introduce into the country a large shipment of drugs known as Trigger, taking advantage of a Mascot Idol festival at the SS Mall in Super Minami, Osaka; and then distributing it camouflaged in food boxes. The infiltrated agent Monika manages to find out the Drug and the main suspect, a villain known as Number 6. Fat Gum and the police enter the loading dock and order everyone to freezer, but the villain quickly enters a code on a device that makes the cargo mechs to start rampage the place.
After rescuing the trapped operators with the help of Monika, Fat Gum use his enormous strength to pulls out the clamps and leaves the mechs deteriorated. However, Number 6 was prepared for that setback. The damaged mechs approach the batch that containing Trigger and explode, destroying all evidence. The villain takes advantage of the distraction to run away.
While the police force goes after Number 6, Fat Gum decides to take Monika with him to deal with the last remaining Mecha, which is still causing troubles. When they arrive at the place, Monika manages to stop the machine and rescue the person who was trapped in it. However, as happened with the other mechas, the machine starts its sequence of self-destruction. Fat Gum jumps to block the explosion, absorbing all the kinetic energy with his Fat Absorption Quirk. Once everyone is safe, he releases all the energy, getting nobody to be hurt at the cost of his protective fat.
Although there have been no injuries, all the evidence was destroyed, and the villain managed to escape.
Weeks later, after police investigations discovered that Villains Factory's possible base of operations is at Onomura Pharma Corp., Naomasa again contacts Fat Gum to work with Eraser Head and Monika, first to investigate people connected to the company,  and later to participate in the police raid on the pharmaceutical company. The police raid takes place, arresting the suspects and confiscating all possible evidence. They hardly have any problems at first, thanks to Eraser Head and Fat Gum, who defeat the main threats, but when they arrive at asecret laboratory, they meet Number 6, surrounded by creatures known as Bombers. Before they can stop him, Number 6 blows up the labs, fleeing with five Bombers to the Tokyo Sky Egg, with the intention of killing Captain Celebrity.
Fortunately, Fat Gum and the rest of the police operation survive the explosion, and quickly heads to the Tokyo Sky Egg to help. The attack of Number 6 is avoided thanks to the intervention of All Migth, and there is no fatalities. Naomasa introduces Fat Gum, Monika and Eraser Head to Toshinori Yagi, which is the true identity of All Might although that is something they do not know.
At some point, Fat Gum accepted Tamaki Amajiki to work at his agency. Although Tamaki proves to have great potential as a hero, his lack of spirit prevents him from standing out. Fat Gum tries to help him in this aspect. After a long time, following Mirio's advice and with the help from Tamaki, Eijiro ends up interning under Fat Gum as well.
While patrolling the streets in Esuha Market, a group of criminals appear and Fat Gum stops them from running. When one of them escapes, Suneater turns his fingers into tentacles and stops him from fleeing. In a crowd of civilians, another thug takes out a gun, wanting to help his allies. Fat Gum notices the presence of the thug and yells at everyone to get down. The thug fires a special bullet, one that hits Suneater and another at Red Riot's hardened head. Red Riot, now angry, states he will capture the thug. He hardens his whole body and leaps into action.
Suneater tries to use his Manifest again but for some reason he can not activate it, which surprises Fat Gum and questions if the thug has a Quirk eraser. Meanwhile, Red Riot chases the villain to a dead end. The villain uses his Quirk to create blades on his body but Kirishima knock the man down. In this situation, the villain injects himself with a Quirk booster, that allows him to generate unleashes a flurry of blades that consume the entire alleyway. To deal with him, Red Riot hardens his body even further, revealing his new super move: Red Riot Unbreakable. Thanks to this, Red Riot breaks through his attacker's assault and defeats him. The villain tries to escape but Fat Gum intercepts him and traps the villain inside his stomach.
Then, while dialoguing with his interns, Fat Gum states that the villain used an inferior version of the Quirk booster drug he had encountered before. However, internally Fat Gum is worried, because it is the first time he knows the existence of a drug that cancels the quirk.
A few days later, Fat Gum, Eijiro and Tamaki attend a Hero Team Up meeting in Nighteye Agency about the yakuza Shie Hassaikai, and his leader Kai Chisaki. There they are also Izuku, Ochaco, Tsuyu, Mirio and Nejire, Ryukyu - Ochaco, Tsuyu and Nejire's work-study mentor-, Shota Aizawa, Gran Torino, and many other heroes. Once they are all in the conference room, Sir Nighteye explains the situation and comments that thanks to the information some of them provided him with, he was able to make substantial progress with his investigation.
At the meeting it is revealed that Shie Hassaikai has been behind several criminal activities, such as robbery and drug trafficking. They are also responsible for the production and circulation of Quirk-destroying bullets. However, the bullet used on Tamaki seems to be a prototype since it did not destroy his quirk, they tries to obtain the final version. Fat Gum reveals that, after recovering the bullet that bounced off Eijiro, they were able to examine the contents and found out that it contained human blood and human cells. Therefore the drug has a human origin, from someone capable of erasing Quirks.
Then Sir nighteye explains two important details: First, Kai Chisaki, the leader of the Shie Hassaikai, has a Quirk called Overhaul. Chisaki's Overhaul Quirk gives him the ability to disassemble or restore objects, an ability that can break down something and then reassemble it. Secondly, apparently Kai has a daughter named Eri, although there is no birth certificate of her and details about her are unknown; Izuku and Mirio encountered her on their patrol and she had a number of bandages on her. With this Sir Nighteye concludes that Kai Chisaki is probably using his daughter to make those Quirk destroying bullet, disassemble and reassemble her constantly to obtain the raw material. Fat Gum, like everyone else, is disgusted with Kai's crimes and demands that they hunt them down.
After explaining a few more details, Sir Nighteye concludes the meeting, and begins to organize the police and hero operation to dismantle Shie Hassaikai. Sir Nighteye states that their objective will be to save Chisaki's daughter. 
Finally, several days later, the police raid on the Shie Hassaikai base begins. As they go through the operation, several comrades are left behind to face the threats, like Ryukyu's Team against Rikiya Katsukame. The group led by Sir Nighteye advances defeating all Yakuza who try to stop them. As the Heroes head down towards the basement but are surprised to see a wall before them.
Mirio Togata uses his Quirk to phase through the wall and tells the Heroes that the wall was thrown in the way to block them. Tamaki surmises that Chisaki is stalling them for time to hide everything and escape. Mirio decides to go on ahead and phases through. After Izuku and Kirishima knock down the wall, the floor starts to warp and open up beneath the Heroes due to Joi Irinaka's Mimicry Quirk, causing the heroes to fall down in a hall where three members of the Eight Expendables await for them: Toya Setsuno, Yu Hojo & Soramitsu Tabe.
As Fat Gum prepares for battle, Tamaki decides to battle the three Eight Expendables himself in order to make up for falling under pressure. Tamaki restrains and disarms the three yakuza and tells the others that he can handle them. Fat Gum instructs everyone to get moving because Tamaki surpasses them all in terms of ability. Despite his words, the battle against the three Yakuzas turns out to be very hard for Tamaki, suffering numerous wounds during the fight and being about to be killed, but in the end he manages to prevail and defeat them. Tamaki restrains the villains and poisons them enough so they can't pose any more of a threat. He moves to regroup with the others, but he notices the great pain coming from his face and falls unconscious.
Making their way through the underground labyrinth, Fat Gum overhears Izuku and Eijiro discussing Tamaki. He tells the two that the mark of a true man is to have the ability to completely trust someone who has their back. Suddenly, Shota is attacked by a piece of manipulated wall, courtesy of Mimic, but Fat Gum shoves Shota out of the way, and is thrown into another room. As Fat Gum gets up, he is surprised to see that Eijiro is with him. Eijiro tells Fat Gum that he was also trying to protect Shota as well but once he did, he was swallowed by Fat Gum's Quirk.
Suddenly, two of the Eight Expendables, Kendo Rappa and Hekiji Tengai, appear. Eijiro activates his Red Riot Unbreakable, but Rappa’s rapid bullet-like punches are so powerful that manages to break through his defenses. Fat Gum counters with a punch of his own, but is surprised that his attack strikes a barrier instead, created by Hekiji. Despite this beginning, Fat Gum tells Eijiro not to give up and encourages him to keep fighting.
The fight continues, and Rappa launches an assault on Fat Gum with his multiple fast punches, but Fat Gum withstands the attacks and taunts Rappa into a fight to determine who is stronger. Fat Gum absorbs the punches and takes in a lot of energy, which burns away his fat and making his defense weaker. Eijiro watches the fight in disgrace as he thinks he is unable to do anything. However, he is ashamed of his thoughts and appears in front of Rappa, blocking his punches. Although Eijiro is defeated shortly, he gives Fat Gum the time needed to release his accumulated strength. Thanks to Eijiro few seconds of bravery, Fat Gum has absorbed Rappa's accumulated blows which have in return given him great strength, and tells the yakuza that they are going to lose because they underestimated Eijiro. With his spare hand, he unleashes a powerful blast that smashes both villains. 
Despite his powerful blow, Kendo Rappa stands up but to Fat Gum surprise, he does not attack them, if fact he tells Fat Gum that there are first aid supplies in a nearby room and tells him to treat Eijiro's wounds while referring him to as a man. Fat Gum questions about it being a trap, but Rappa denies it, explaining he vows his interest in them both and wants fight them again, despite Fat Gum Fat warns him that he will be arrested. Fat Gum moves Eijiro to the infirmary where he bandage him, stopping the bleeding. Then he asks Kendo what he was doing working for a gang. After hearing his explanation about Kai Chisaki, Fat Gum begins to become concerned and proceeds to ask Rappa about Chisaki's plans. Due to their injuries, neither Eijiro nor Fat Gum can continue participating in the rescue operation. 
Meanwhile, the raid continues. More and more members of Shie Hassaikai are being defeated by the remaining heroes. After being unconscious for several minutes, Tamaki wakes up when he feels that something bad happened to Mirio immediately after the latter gets shot with one of Overhaul's Quirk-Erasing Bullets.
Tamaki goes out in search of others, encountering a wounded and unconscious Mirio and several police officers who accompany him. While carrying Mirio, he saves Shota's life when he was about to be stabbed by Hari. Tamaki manifests his hand into a swordfish and pierces Hari's arm with the bill, causing him to drop the sword. The Police Force who demand that Hari surrender. Tsuyu Asui appears from behind and asks Tamaki to come with her to the surface.
Once there, he discovers that Izuku has managed to defeat Overhaul and rescue Eri. With this victory, the members of Shie Hassaikai are arrested sending to prison. Fat Gum, Eijiro Kirishima, Tamaki Amajiki and other wounded heroes are sent to a hospital to be cured and recover. Although they suffered serious injuries, none of the wounds is life-threatening and they recover over time.
With the beginning of October, the three go to Sir Nighteye's funeral, who died because of the injuries he suffered in his fight against Overhaul, alongside Sir Nighteye's employees, Ryukyu's group, All Might and Eraser Head. The Work-Studies program is put on hold.