"Promise" (約束 Yakusoku?) is the 70th chapter of the Toaru Kagaku no Railgun manga series. The chapter features the story of Misaki and Mitori liberating Dolly from her bondage. It is the final chapter of the Railgun side of the Daihasei Festival Arc.
The chapter was not serialized in Dengeki Daioh and only released for the tankōbon. It was numbered Chapter 70 due to Chapter 65 merging three magazine chapters into one.
As Misaki and Mitori enter the building where Dolly is kept, Misaki tells her that Dolly perceived her as Mitori, as at the time she still didn't have the ability to manipulate memories. Misaki says that at some point, Dolly noticed that she was a fake, to which Mitori notes that Dolly had surprisingly good intuition. Mitori then says thanks as the elevator comes to their intended floor. She says to Misaki that with her at the end of Dolly's life, at least she wasn't alone, and says that was her biggest regret. The door opens to reveal the capsule where the clone of Dolly is kept, and runs up to it. The clone's consciousness wavers until finally it awakens and calls out to Mitori's name at the sight of her. There, after a terrible long count of years, the two friends, Dolly and Mitori are finally reunited. Dolly, seeing Mitori, asks her if she is Mi-chan, to which she confirms. Breaking down in tears, Dolly says she missed her and the two girls embrace. Dolly says that she thought Mitori hated her because she found her creepy, to which Mitori denies. She then notes that she remembers Mitori's scent and notes on how nostalgic it is. She then says that she loves her.
Meanwhile, Misaki, who is hiding in the shadows watching them, finally tries to leave. However, Dolly calls out to her, knowing who she is. Misaki tries to play it off, but Dolly says she knows she is Misaki because they are always together in their dreams, and says the time they've spent together is the only memories they both have. Misaki is at loss for words, and then finally, she apologizes to Dolly. With emotions overcoming her, Misaki says that she is sorry for deceiving her, manipulating her memories, and her getting lost in the comfortable lie of being her friend. She says that if she had looked into who "Mi-chan" was then she could've reunited her with Dolly, and if she had known the truth of the experiment she is sure that Dolly wouldn't have died. Misaki says that it is all her fault for selfishly putting herself between the two of them. And then states that she'll do anything to make it up to her.
Then, Dolly with Mitori, come from behind Misaki and embraces her, telling her that she wants her to take her to the beach, let her ride a roller coaster at an amusement park, let her see a hippopotamus at the zoo, ride an airplane, and see an aurora in Finland. She continues on with many more requests, to which Mitori says that it is way too much. But Dolly can only help but laugh. She says, as she leans on Mitori, that if they are all together, anywhere is fine, and asks if they can do that. Hearing this, Misaki says that Dolly still ignores what everyone says and force her opinion on them. She turns to them to reveal the tears upon her face and says that Dolly is selfish and that there's no arguing with her.
Back with Mikoto, she checks up on her Sister, Misaka 10032, declaring that her fever is down. Misaka 10032 laments that the festival ended while she was lying in bed. Mikoto says that she shouldn't sulk, and tells her that she should take her somewhere at some point, and asks where she would want to go. Misaka 10032 immediately answers the beach, to which Mikoto says that the beach is season has ended. Misaka 10032 says that she doesn't know why, but says that she wants to make an exciting promise with someone important with her, describing it as a sentimental sense of deja vu.
- Misaki and Mitori frees Dolly and reunites with her.
In order of appearance:
- Mitori wears a band-aid on her nose, a reference to the fact that Kuroko hit Mitori's nose during her fight in the sewers back in Chapter 66.
- Dolly telling the girls that she wants to go to the beach as well as the fantasy at the end of the chapter is a reference to her wondering about the ocean back in Chapter 60 and Chapter 68.
- Mitori: "Thanks. With you there, she wasn't alone at the very end. That was my biggest regret."―to Misaki about Dolly.
- Misaki: "I'm sorry for deceiving you. I'm sorry for not catching on to the lie. I'm sorry for butting in on your memories. I'm sorry for butting in on your memories."―to Dolly.