Magia Record Story Otsukimi is After an Elegant Tea Party
The following is a summary posted on 4chan's /pmmm/ general.
(This story happens after the first arc, when the original Magius has already disbanded)
It's Moon Viewing Tea Party today at Mizuna Academy and not an ordinary one, but the school is also receiving guests today from the most elite school in the city! But Yukika Nanase has been allowed a day off, despite being a member of a student council because of an important family dinner. She decides to visit a game center on her way home, when suddenly she gets teleported back to the school where everyone is preparing for the party. (This is due to her magic, where it causes her to be drawn to troubles only she can resolve). By now, Yukika is used to it enough and decides to go with the flow of things.
The plan was to meet the guests, hold a tea ceremony, have Tsukuyo Amane play the flute and watch the moon. It seems simple enough but where Yukika is, there's bound to be trouble. Tsukuyo gets a message that someone in her family is sick, which is why she cannot participate. Yukika resolves it with the help of the tea ceremony club. After that she finds a bag with a White Feather robe in it and a note from Yukika to Tsukuyo. The problem is, Yukika doesn't remember writing it in the first place, nor does she know why the robe is there.
Guests are starting to arrive in two groups. The first one is led by Hotori Yuzuki and Rion Yuzuki. The second half,led by Yuuna Kaharu is late, so they decide to start without them. But then, the pipe in the old dorm bursts and tea ceremony club gets flooded, so Yukika has to swiftly find a replacement for the third and fourth parts of this evening's program, which she does with the help of the ikebana (flower-arranging) club. At the same time,she can't help but notice how she cannot actually go into the old dorm.
Yukika tries to go to the old dorm on purpose, but meets Hotori and Rion on the way who sense a familiar there. She decides to help, but for unknown reasons transforms into a White Feather robe instead of the usual outfit. After the battle, they tell her they were Black Feathers in the past and were saved by a White Feather once. They think it was Yukika, but she doesn't have time to argue,since the ceremony had started already. During it, Yukika accidentally tells about the robe Tsukuyo forgot at Mitama's and she has to return it. She leaves the room to catch a break and feels her memory getting jumbled, because she remembers taking the robe to put it in Tsukuyo's locker and going to the game center simultaneously. Yukika comes to the conclusion that there are 2 alternative timelines happening at the same time and overlapping each other. She goes back to the old dorm, sees a Witch barrier and...the building comes crushing down, because Yuuna is landing her helicopter nearby.
Inside the barrier Yukika finds Kyubey (who snuck into Kamihama when Even was down, but became stuck when the small Kyubey sacrificed himself and reinstated the barrier), and he finally explains what is going on. Yukika was right,there is a singularity created by the two timelines, caused by Rion's wish in the original timeline. In the original timeline, Rion was not a meguca, only Hotori was. And she was also a Black Feather in the past, and she kept all of this a secret. In the original timeline, Yukika went to the game center, Hotori found a Witch in the old dorm and Rion followed her. The Witch had become very powerful due to all the negative emotions in Kamihama, which was still recovering from the terrible disaster. Hotori became overwhelmed, and wasn't able to fight and protect her sister at the same time, when someone in White Feather robes appeared, allowing the twins to escape, but she lost and was killed. The Witch's attack was powerful enough to go outside the barrier and damage the building, causing it to crash on the twins with Hotori dying while protecting Rion.
After Hotori dies, Rion makes a contract with Kyubey who was inside the barrier, wishing to prevent the building crushing, but she doesn't have enough potential to completely make it so, since it's a fixed point in time. Instead it creates a second timeline and pulls Yukika out of the original timeline due to her magic making her a perfect choice. To fix history she has to take the place of the White Feather and destroy the Witch.
Yukika is looking for the exit, but instead finds Hotori and Rion. She realizes that the three of them won't be able to survive and tells them to leave while she decides to stay and accept her fate (just like the White Feather did in the original timeline). Also, the only Kyubey in Kamihama gets killed taking a hit for Yukika.
She has no regrets helping others in need, though she is frustrated and upset that she is unable to fulfill the White Feather's last wish which is to defeat the Witch, and wasted Kyubey's sacrifice for her.
She fought bravely, but is eventually overwhelmed, coughing out blood. However, her emotions exploded and she releases her doppel (which she thought didn't work anymore), finishing off the Witch. As a result, the two timelines merged. Rion is a magical girl, Hotori is alive and not a Black Feather, and Yukika is the only one who remembers what happened.
In the end, she is able to see the moon with the council members of the two schools, and the twins grew closer to her even though they know now that she is not their original savior.
Sometime after the party, Yukika leaves Kamihama to seek out and talk to a Kyubey from the outside and tells him the events of the singularity. She's dismayed to know that even the other Kyubeys don't know what happened to the Kyubey that was inside the barrier other than it is dead. Kyubey explains that they will stop conducting experiments in Kamihama since there is no way to know what happens, and Yukika is now the only person to remember the White Feather and that Kyubey's sacrifice. It is amusing to note that Kyubey sees the doppel system as weaponizing the taint from Soul Gems. This, and the fact that Yukika's innate makes her more powerful when the odds are against her is why she was able to win. She admits that knowing the other side of the world, while exciting, is not what she expected. She then gets teleported again into some strange barrier, where she meets Hotori and Rion again. They tell her that Yuuna is in trouble.