As Mayushii organizes a huge Christmas bash to cheer up Suzuha, Okabe is finding Amakurisu’s constant calls a nuisance. But she insists he keep his promises to her by showing her the lab. While her first impression is that it’s a dump, she admits she always yearned to share a room with people, with “forks and spoons and stuff.” She already feels so much like Kurisu, it’s almost as if Okabe’s Facetiming the real thing.
As Suzuha is visited by her benefactor Feyris to try to convince her to come to the party, all Suzu can think about is how to convince Okabe to accompany her to the Steins Gate.
At the Brain Institute, Leskinen is excited to see what comes of Okabe convering with Amakurisu, while Hiyajou is more weary, thinking of displacement behavior and knowing firsthand how easy it is to replace the Kurisu she lost with Amakurisu in her mind. Leskinen has an answer for that as well: go out with Okabe for Christmas Eve.
The next day Leskinen and Hiyajou meet with Okabe, but before they do, Okabe and Amakurisu have a crucial conversation on her true nature as an “incomplete AI” due to the fact everything she sees and hears is logged, meaning she can never “forget” by anything resembling the same methods real humans forget, which is not so much about missing information as information that slowly changes over days, months…or decades.
Along those lines, Okabe and Hiyajou are at the mercy of their memories of Kurisu, which are constantly meshing with Amakurisu, to the point she feels more and more real.
Hiyajou looks the tiniest bit disappointed when Okabe says he has plans for the Eve (even if she likely wasn’t going to take Amakurisu or Leskinen’s advice and ask him out), but Okabe presents a third way: he invites them both to the party.
Everyone ends up turning up—even Suzu, whom Daru fooled and who nearly killed her mother when they all surprised her—and it’s a jolly good time, bringing back life and vitality to the lab. They even get in another “Hiyajou is kid-sized” gag when Nao asks her what grade she’s in, while Leskinen mistakes Ruka for a girl (though he’s dead sexy regardless).
Okabe’s participation in the party is interrupted by a call from Amakurisu, which he goes to the roof to answer. When he takes a while, Mayushii follows him up there, and after overhearing him talking with someone (whether she knows it’s Kurisu), heads back to the lab. Poor Mayushii, who had to tell her cosplay buddies that despite how close they are, Okabe likes someone else…without mentioning that someone else is dead, of course.
One thing Amakurisu will never forget is that Okabe called her Christina when they first met, and one of her three theories is spot on: it’s the name he used to call the real Kurisu (whom she calls “Original Me”). She didn’t think that theory was likely because of the way she knew she’d react, and says the exact same words she always said to him, and in the same way. He confesses he used to call Kurisu that because he was too embarrassed to use her real name.
The problem is, he’s talking to Amakurisu as if she were Kurisu. Escaping a Santa costume photo shoot, Hiyajou, who came to the roof to check on Okabe right after Mayushii, snatches his phone from him and turns off the app. The Amadeus app. Not a Facetime call with Makise Kurisu, whom she tells Okabe “isn’t here anymore.”
This upsets Okabe greatly, as if suddenly lifted from some kind of spell, and triggers a torrent of muddled memories from other world lines. Hiyajou may have thought Okabe was just another good friend of Kurisu’s who’d be all-too-easily deluded by Amadeus, but there’s a lot more going on than she knows or would quickly believe.
Like, say, the fact that Suzuha is from the future and her father is inventing a time machine.