gentrychild answered:

kepIzuku’s mom has to leave for work (because she would never sell her son’s soul) but he never has the time to finish reading the note she left him because he is kidnapped by AFO into the demon realm 0.02 seconds later, and AFO is so happy and declares that Izuku is his heir, and that’s in the contract, and he should chose a quirk before the rest of the demons realize he is human and eat him.

After three impressive attempt escape (Demon Lord AFO doesn’t even understand how Izuku managed to actually escape the palace despite not having a quirk (what do you mean, he used a rope or hid inside a suitcase???) and discovering that Inko did have her agreement as she was abroad and needed someone to take care of Izuku (Izuku himself cries because his poor mom was obviously fooled), so he just… resigned himself to his fate of being spoiled rotten.

Izuku then goes to the famous demon school UA, where everyone is extremely competitive, belonging to different faction, and Izuku just shows up with his only goal not to be eaten, and since it’s Izuku, chaos somehow erupts and he sometimes gains a reputation of being an utter badass despite the fact he doesn’t have one iota of magic and he just keeps improvising. Not long after arriving to school, he meets this legend of a demon, All Might, who was cursed so he can’t use his power, so he is desperate for a successor. He finds Izuku, who is human, but damn, his master, Nana, also came from the human realm, so that won’t stop him.

So, when Izuku goes back home, that day, you get that exchange:

AFO, Demon Lord firmly opposed to the faction All Might represents: “Izuku, what do you have here?”

Izuku: “One for All!”

AFO: “NOOOOOO!”

Kurogiri: “Where did he even find One for All?”

But it’s too late.

Also, it’s good to realize that AFO belongs to the chaos faction, is terrifying, has a whole court of demon just as terrifying, and Izuku just barges in “Hi, I guess I’m his son, don’t mind me!” with the most adorable baby face ever and people are very suspicious until Izuku does this fun thing when he passes from Baby to IT’S MURDER TIME in 0.02 seconds flat and everyone is “Yeah, I see the resemblance, he is terrifying.”

The plot resolves around school adventures, demons being always willing to fight each other, and trying to find who cursed All Might because for once, it wasn’t All for One.

(Like… They keep trying to kill each other but people are used to it and just duck by now.)

Also, at some point, Inko (by phone) asks Izuku if he is getting along with his father and Izuku is “Wut?” and it’s finally revealed in the most anti-climatic way that AFO is his dad and how it was mentioned in the note Izuku never had the time to read. AFO freaked out when Izuku didn’t recognize him and just Never Mentionned It Because Inko Must Have Been Ashamed Or Something??? and Izuku is “????” at his mother and keeps repeating, dumbfounded “I can’t believe you kept telling me he was working abroad and how it was Hell and I just assumed he had a difficult job.”

Kurogiri, who did NOT know either: “When you declared that this human was your son, I thought you were just adopting him to have a heir???”

AFO: “Why did you think I kept going to the human world 15 years ago?”

Kurogiri: “I assumed you were just doing your best to ditch the paperwork???”