Following their internships, everyone has returned to school, and All Might decides to tell Izuku the truth behind the Quirk, One for All. This is how it goes:
Way back when, during the first generation of Quirks, there was a man who won the Superpower Lottery. His quirk, 'All for One', allowed him to give and take Quirks as he saw fit. He became a villain and started a reign of terror so bad that even now, Japan wants to forget it and deliberately doesn't teach it in schools. This man had a little brother who lost the Superpower Lottery, and was thought Quirkless. The little brother had a far more heroic nature than All for One, but couldn't meaningfully oppose him, being frail, sickly, and Quirkless. Then All for One gave him a Quirk, and everything changed.
Unbeknownst to All for One, it turns his brother did have a Quirk, albeit a rather useless one they never knew existed; he could transfer his own Quirk to others. While completely unhelpful on its own, when merged with the new Quirk, which allowed the user to stockpile power, it became something more: One for All. The Quirk that could be passed on, kept increasing its power through the generations, and couldn't be taken against the user's will.
The first user of One for All died a long time ago, and never defeated his brother. But he did pass on his Quirk, and the eighth user, Toshinori Yagi, was able to finish what he started and kill All for One. Or so he thought, until Nomu showed up. The Nomu had multiple Quirks that weren't originally his, a sure sign of All for One's involvement. All Might never told Izuku this story because he thought it wouldn't be important, but now that All for One is back, he thinks Izuku should know about the powerful supervillain probably gunning for him.
Tropes:
- Aren't You Going to Ravish Me?: Jiro is just as disgusted as the other girls for hearing Mineta talk about them while attempting to use a peep hole. She deservingly pokes his eye with her jack, but she appears offended at him not talking about her at all.
- Call-Back: When asked if Izuku remembers what All Might said to him back at the start of the series involving One For All, he remembers being given a piece of his hair to eat, and the hero saying to "EAT THIS". When he imitates his face and voice, the annoyed All Might clarifies the time he said One For All must be surrendered willingly for the transfer to work, like with him and his previous mentor. Both of these did occur last season.
- Create Your Own Hero: All Might reveals that his and Izuku 's Quirk "One For All" was created by a villain who could give and take Quirks. The villain gave a "Stockpile" Quirk to someone who unknowingly had the Quirk to pass along it to other people, which led to One For All's formation. The users of One For All all opposed the villain, and the villain was eventually defeated by All Might.
- Eye Scream: Mineta finds a peephole that is between the men's room and women's, and takes full advantage of it to try and peek at the girls... before Jiro stabs him in the eye with one of her earlobe jacks.
- Evil Is Cool: In-Universe. Kaminari thinks about Stain's conviction and tenacity being a part of his cool factor, until Midoriya reminds him that the same guy nearly killed his friends. Iida at least understands, but doesn't excuse his bloodlust.
- Facial Horror: At the end of the episode All For One appears in the flesh for the first time and he is shown to have lost most of the upper half of his face as a result of his battle against All Might five years ago. The doctor working on All For One even notes that, if they had found the Super Regeneration Quirk earlier, they might have been able to fully heal him but since his face has scarred over there is nothing they can do.
- Fantastic Racism: It's shown during All Might's story that back when Quirks first started appearing people reacted negatively due to it no longer being clear what it meant to be human, not unlike X-Men. There are even protesters shown with signs rallying against people with Quirks.
- Fling a Light into the Future: All For One's brother realized that he could never oppose his brother on his own so instead he transferred the Quirk to someone else hopping to entrust it's power to the next generations. One For All has been passed on ever since with the various wielders of the power tasked with eventually defeating All For One.
- Foreshadowing: All Might warns Izuku that the boy may have to deal with the same "great evil" as he and his his predecessors once did, implying that he may fight All For One. All Might also believes he may not live long enough to see the fated battle of his protege.
- He's Back!: For better or worse, Iida returns to his Straight Arrow attitude. Some of his classmates aren't too pleased.
- Internal Retcon: It's indicated that All for One's reign of terror was such an unpleasant chapter for Japan that these days, decades later at least, it's not taught in history classes and has been reduced to the stuff of online rumors.
- Origins Episode: The episode is dedicated to All Might explaining to Izuku the origins of One For All and its entwined relationship with the Big Bad "All For One", a feud that has continued to the present day.
- Your Days Are Numbered: It's implied at the end that All Might's time is running out, due to his injuries. When Izuku says that he can handle the potential fight ahead against All For One because All Might will be with him, All Might hesitates to say something important and can't bring himself to do so. At the end of the episode All Might thinks to himself that, by the time Izuku faces All For One, he won't be there for his protege.
