We start with a flashback. Kurogiri warped Aizawa and Mic to some random tiny island. Mic is about to beat up Kurogiri, but he gets stopped by Aizawa, who notes that the Noumu is crying. Mic says they shouldn't expect another miracle to occur like at Tartarus, but Aizawa replies that there had to be a reason why Shirakumo ended up saving their lives.... they seemed to have reached his humanity somehow. As further confirmation of Aizawa's theory, Kurogiri manages to utter Mic's real name, Yamada.....
Aizawa gets in contact with Tsukauchi for a status update. Monoma is out of action due to being KO'ed, but there are still several heroes remaining who can still fight. Thus, Aizawa puts his plan into action! Using Kurogiri's portals, he gathers every able-bodied hero he can (interestingly, one of them is yet another cameo from a past work of the author's.... Astro from Barrage/Sensei no Bulge). Back in the present, he asks Deku how long ago he lost his arms, but the young hero doesn't know. That's when the teacher presents him with a new gift....
Eri's horn.
In another flashback, we see when the girl gave her horn to Aizawa. She knows she can't enter the battle, so she decided to lend support by asking Ectoplasm to cut off her horn. Aizawa warns her that doing this could potentially damage her quirk beyond repair, but she doesn't care.... she felt the need to help in any way she can. Not just for Deku, but for everyone fighting, including All Might and Kacchan. She also reveals her new dream.... to be a singer like Jirou. Back in the present, as Deku receives the horn, we learn that he should be healed in an estimated 2-3 minutes, and is told by Aizawa that he must live to hear Eri sing. As the healing process begins, his classmates step out of a portal to back him up against All For One....!!!!