Inside the building contains Sir Nighteye's personal office that is decorated with multiple posters of All Might, as well as his main desk where his laptop sits and a large bookshelf. On the second floor, there is a large meeting room.
SIr Nighteye's career as a hero begins when he broke his association with All Might due a disagreement. After his bloody fight against All For One, All Might was seriously injured. Nighteye asked him to look for a successor and retire, but he refused because with his absence the crime would gain strength. Sir Nighteye threatened to stop helping him if he continue his reign as the No. 1 Hero, because he predicted with his Foresight that if he continued on that path, in six or seven years he would have a death too horrible to describe at the hands of a villain. Despite the warning, All Might ignored Nighteye's pleas and they dissolved their partnership completely.
After this dispute, they went their separate ways. Nighteye started his own hero career, opening his own Hero Office, and hired Bubble Girl and Centipeder to work with him as sidekicks. Despite his breakup, Nighteye doesn't hold any resentment or ill-will against All Might, decorating your office with merchandising of him. In fact, the objective for which he opened his own hero office was to find a way to prevent his prediction about All Might's death from being fulfilled.
He sought the right candidate to be the new heir of the One For All and, in this way, replace All Might as the new symbol of peace. Sir Nigheteye thought he found the right candidate in the young Mirio Togata, however Toshinori informed him that he had already chosen a Quirkless young boy to be the ninth user of the One For All. Sir Nighteye was against this decision, and took it upon himself to cultivate Mirio Togata, whom he believed to be the truly appropriate successor. Thanks to his training, Mirio became in one of U.A. High School's most powerful students and a contender for the No. 1 spot among Pro Heroes.
The Nighteye Office begins tailing a Yakuza organization called the Shie Hassaikai after know a peculiar incident that involved Kai Chisaki, also know as Overhaul, the leader of the Yakuza, with a gang of thieves called the Team Reservoir Dogs. After gang stolen the cash register of a store, ran into Overhaul in their flight.
The incident ended with "zero casualties". After feeling a sense of pain and loss of consciousness, the gang came out of the incident completely unscathed; not only that, all their chronic illnesses from rheumatism to cavities were all healed without a trace. The reason for this phenomenon is believed to be due to Overhaul's Quirk, but as a result, since there were zero wounded and the gang was apprehended, Overhaul could not be charged with any crimes. The money that was stolen was the only thing that completely vanished without a trace to which the Police Force believed that no crime had occurred.
Despite the Police Force labeled it as an accident, the Nighteye Office found strange since it didn't make any sense and decided to investigate the Shie Hassaikai. Centipeder started investigations regarding the Shie Hassaikai and found out that they have been contacting other villainous groups and involvement with black market groups all over Japan; their motive for doing so is to increase their organization and finance. Soon after the official investigation began, the Shie Hassaikai made contact with the League of Villains, and both organizations meet in a abandoned factory building a meeting that ends in confrontation in which at least one person dies.
Shortly after starting the investigation, Izuku Midoriya, the boy All Might had decided to transfer the One For All, asks to do his Hero Work-Studies at Nighteye Agency. Once Izuku shows up at his office, Sir Nighteye decides to challenge him, and during the same, he tells Izuku that All Might made a mistake in choosing him and Mirio was the true candidate to inherit the One For All. Although Izuku fails the challenge, Nighteye is impressed by his demonstration and hires him, claiming he planned on employing Izuku all along, but he still not acknowledge him as the proper successor. He reveals that the reason why he takes Izuku is to make him realize who truly is the worthy successor of One For All by having him work alongside the pros.
With Deku working at his agency, the next day Nighteye explains the investigation he is carrying out on Shie Hassaikai, and sends him to patrol the streets next to Mirio, while he and Bubble Girl watch over the Yakuza's hideout. As a result of chance, Deku and Mirio stumble upon a girl full of bandages who fled scared of Overhaul. The leader of Shie Hassaikai appears, and tells them that the girl is her daughter Eri whom he had just scolded.
The boys know who he is and suspect that he has been done something to the girl, but Mirio tries avoid creating any problem with Kai that could ruin Nighteye's investigation, and allows him to leave with the girl, but Deku does not want to give Eri back after check how scared she is. Overhaul says he will explain if they follow him down the alley. When he is about to remove one of his gloves, Eri runs back to him. Overhaul apologizes for any problem and wishes them good luck as he leaves with Eri.
Mirio stops Izuku from going after Eri while commenting that Chisaki used his killer intent to make Eri listen to him. Later, Izuku and Mirio meet with Nighteye and Bubble Girl and inform them of everything that happened with Kai and Eri. Izuku expresses his desire to save her but Sir Nighteye reproaches him to not be reckless, he must plan his actions carefully to always obtain the best results.
A few days later, Nighteye holds a meeting in his offices with the Work-Studies gang and several heroes like Eraser Head, Gran Torino, Fat Gum, Ryukyu, and many more. He reveals that he called for this meeting to share information as well as to discuss how to proceed in dealing with the Shie Hassaikai.
From all this, it is concluded that Shie Hassaikai has the capacity to produce a substance that allows to eliminate Quirks. At the moment it only sells prototypes of this drug that cancels Quirks temporarily, but it does not doubt that soon Overhaul will get the definitive version that eliminates them permanently. The key to the production of this drug is the girl Eri, from whose blood and cells Overhaul obtains the essential material to produce the drug. Nighteye concludes the meeting by clarifying that their objective will be to dismantle Shie Hassaikai and rescue Eri.
After several days of investigation and preparation, Nighteye begins the raid to the Shie Hassaikai's Headquarters with a combined force of Heroes and police, facing the members of the Yakuza. Sir Nighteye and Fat Gum's groups charged into the mansion and since Kai has not shown himself, they assume that he must be in the basement evacuating everything to hide which includes Eri. The Heroes reach a shrine with a vase on it; Sir Nighteye removes the vase and pushes on a wooden panel, which causes a door to open. The assault team head down towards the basement, but once there, they discover that Overhaul has used their power to block their way with a wall. Mirio decides to go on ahead and phases through.
Mimic, the general manager of Shie Hassaikai, uses his Quirk to modify and alter the corridors, trying to block the rescue team's progress, and separating them to force them to face several members of the Eight Bullets and Himiko Toga and Twice from the League of Villains. Meanwhile, Mirio manages to reach Chisaki, and after defeating Deidoro Sakaki, Shin Nemoto and Chronostasis, he confronts Shie Hassaikai's leader to rescue Eri. Mirio reproaches him for his cruelty and the experiments he performed on his daughter, to which Overhaul replies that Eri is not his daughter, and attacks Mirio with his quirk at full power, starting a fight between them.