Holy Day of the Calamitous Birth

Saiyaku Koutan no Holy Day (災厄降誕のホーリー・デイ) is a short story originally published in the Chaos;Head & Steins;Gate Science Adventure Series Maniacs (カオスヘッド＆シュタインズ・ゲート 科学アドベンチャーシリーズ マニアックス) information book, ISBN 978-4798601465. The story takes place after the true end of the visual novel.

In December in Akihabara, Rintaro Okabe is suffering from PTSD due to the events of the summer when he receives a letter from Kurisu Makise on December 14th, his birthday. However, things start to go wrong when Nae Tennouji suddenly goes missing on Christmas.

Plot
In December in Akihabara, Rintaro Okabe is suffering from PTSD due to the events of the summer when he receives a letter from Kurisu Makise on December 14th, his birthday. However, things start to go wrong when Nae Tennouji suddenly goes missing on Christmas.

The story begins with Okabe putting on a chuunibyou act because things are going too well in this Worldline (to which the others are either unaware or unbelieving of it). He receives an email anticipated from Kurisu and quick opens his inbox, though to his utter disappointment, it is spam mail.

Hashida mentions how "mad" he is about Okabe being a "Riajuu" compared to himself; an otaku. Mr. Braun barges in to ask that the group of 3 watch his daughter as he and Moeka go to do "business" in another area. Over the course of their supervision for Nae, they consistenly try to cheer her up or get her excited, though none of it works very well. Urushibara joins their group which Nae quickly attaches to and Okabe and others head off to walk around in Akibara.

Okabe begins experiencing PTSD from the time when he was attacked by Nae's 2025 vengeful counterpart (this did not happen in the anime.) after she stares at him with a blank expression before looking away, believing she frowned. Infering she was thinking of something sinister he wraps his scarf around him. They encountered a mini-festival where Okabe, Mayuri, and Ruka took pictures before hearing Nae's minor screams after being touched by a "suspicious ugly guy." Okabe goes to confront the man before a santa from earlier blocks his way. Though as it turns out; the santa is on Okabe's side and stops the man.

The group meets Faris later in the day and asks her if there are any interesting places near the cafe, she mentions a gamecentre and they make their way there. Hashida challenges Nae to a match in any game she wants, surprisingly; she picks a fighting game. Okabe begins over examinating Nae over her one handed catch of the yen he threw to her to test if what he was seeing was real or not. Hashida picks a fat character that Urushibara mentions is similar to Daru's own look. Hashida was extremely confident that he would one-sidedly destroy Nae. It proved false. Nae absolutely obliterates him and mentions that she simply felt her body move on it's own during that, to which Okabe panics and evaluates how she gourged Daru's characters flesh just like she did before to himself but is reminded that everyone has some tiny form of Reading Steiner present in them.

Okabe realizes that his tensed body has caused him to have urgency to use the restroom so he quickly rushes off. He looks through his emails again as to check if Kurisu had sent him an email. He reopens her last email from 10 days ago that had some light words of blessing and talked about how busy her research was becoming. However, an anxiety bloated within his body and his Hououin Kyouma self laughed at him; telling him he was nowhere good enough for a genius girl with a bright future ahead. Okabe suppresses these thoughts and reveals to himself that he simply has the Hououin Kyouma persona no longer for Mayuri, but rather himself. For the sake of keeping his sanity and as a reminder that he is safe in the Steins Gate. He finishes his business in the restroom and exits but is greeted by Nae Tennouji.

Asking if she need go to the restroom herself and exclaims its location Nae quickly responds with, "Hey, Uncle Okarin." As he was about to correct her, she replies again. "I'll kill you. Okabe Rintaro." Okarin quickly falls into disarray and dreads in his mind what is happening, though hastly comes to the conclusion he is simply tired. Okabe and Nae converse about how he looked throughout the day: Tired and depressed. Nae mentions that he has the same eyes her father has whenever he thinks of her mom. Okabe later asks Nae how she was so great at that fight game though she brushes it off as she practices and plays it plentiful with her friends. Finishing their conversing he redirects Nae to rejoin with the group.

Okabe rejoins his group, however Nae is nowhere in sight. It becomes a major problem after an hour she does not appear when called to the front counter. Urushibara receives a call from Nae. They ask for her location where she vaguely describes it but is subdued by a man and the call is ended. Before the call was ended Nae mentioned a symbol that looked similar to a 6 and 9 corresponding to one-another. Hashida is unable to find her with just this much information but Mayuri quickly cracks it by thinking in a more visual way. The 6 and 9 represent the Yin Yang Symbol. Though this alone was still not enough information and Okabe was about to call it off. Until, an illusion of Kurisu to him to stand up and keep going, not to give up. Through the help of Faris, who's tweet sent roars through social media thanks to her influence, viral attackers managed to crack the location of the kidnapper.

Okabe and Hashida battle the massive kidnapper and subdue him with Luka carrying his "Demon Sword Samidare" and Mayuri with the "Cylalume Saber". As it turns out, the kidnapper was the foreign santa from earlier of that protected Nae. He had her dress as Rai-Net characters and take photos of her. As Okabe was in deep thought, he heard the large otaku's groans and turned around to see Nae kicking the man in the crotch 1, 2, 3 times.... and more. He is taken by the police and the situation is handled.

The Lab Mems rejoin one another back in the apartment and have a small buffet for Nae's sake. Mr. Braun returns after having several missed calls but Nae dismisses it as simply wanting to hear her dad. The lab won over his uneasiness. The day is finished.

Later, Okabe is walking outside and greeted by Mayuri, when he receives an email from Makise Kurisu, with a post script stating that she sent a letter to him. However, when he rushed back to his mailbox, there was nothing.