Magia Record Story The Witches' Paradox Extermination Battle
Madoka and friends have traveled to Kamihama to hang out with Iroha, but once they arrive in Chuo Ward, they have a highly unusual encounter with a Witch - one who had abruptly escaped from them months ago, but now appears with all the same wounds on her, as though they'd just fought her hours ago. They contact Iroha, only to find that a sudden Witch invasion has engulfed Chuo, sparking a crisis. After much consideration, Touka and Nemu have concluded that the Witches are coming from Endless Mirrors, and believe it may be possible that the Mirrors are not simply connected to the present, but to the past and future as well - and possibly even parallel worlds.
Iroha and the others investigate to help Touka and Nemu determine the proper course of action. In the meantime, the PMHQ and other magical girls across the city go around fighting witches, with the rule that they cannot use any grief seeds of the witches they slay - because if the witches are coming from the past or from different timelines, then their destruction may risk universe-threatening paradoxes. And since the consequences of a paradox are yet unknown, they'd rather not risk causing one to begin with. In the meantime, Tsukuyo, Tsukasa, and Mifuyu help shut down the mirror portal that the witches are using to travel to Chuo, while the one that comes from the Mitakihara of the past still needs to be found.
As the PMHQ seek out witches, Sayaka gripes about how they can't use any of the grief seeds of the witches they defeat in this gauntlet. Madoka, however, has some spares she offers to Sayaka - mentioning that it's to make up for all the ones Sayaka had given her several months ago. Sayaka, however, has no clue what she's talking about.
Soon, however, the PMHQ encounters a brawl breaking out when two witches start influencing people in the area. After eliminating the witches responsible, they find in the aftermath that a chemical tanker truck has been stolen. Iroha contacts various Chuo girls, like Meiyui, Hinano, and Kokoro, but no one's seen any sign of it...the only magical girls in Chuo that Iroha hasn't spoken with about the issue are Puella Care - which makes her, Yachiyo, and Tsuruno realize that's the only place the truck could be, in the restricted area of Chuo where Puella Care camps out. Madoka, too, recalls a certain witch they fought, who influenced people to make toxic gas and attempt suicide...
Iroha (well, Yachiyo helping Iroha with her map app) sends the location to the PMHQ. They hurry there and find Puella Care trying to fend off a brainwashed crowd of people from accessing the tanker. As the eight of them struggle to figure out how to divide their forces between crowd control and HN Elly, Iroha and friends swing in to provide backup, leaving the PMHQ to fight the witch.
Once in the Labyrinth, Sayaka finds herself exhausted from all the fights she's been through - not just from tanking hits, but because of all the unknown factors involved, like the number of witches they need to defeat (Homura commiserates out loud - she knows full well how not knowing what will happen next can be incredibly stressful). Madoka implores Sayaka to use the spare Grief Seed she offered earlier, which leads Sayaka to question Madoka about that incident she mentioned earlier. No matter what Madoka says though, nothing jogs Sayaka's memory, and as Madoka tries to think of what specifically Sayaka had said back then, HN Elly emerges, and they all concentrate on taking her down.
After the fight, they all worry about how many Witches are left to defeat - until Madoka has a sudden realization: they have, in fact, defeated all the Witches! Collectively, all the magical girls have gathered seven Grief Seeds - which is the exact number Sayaka gave her before. Madoka and Homura realized that Sayaka must have gone back in time to preserve causality, by bringing the Grief Seeds back in time to Madoka to indicate how many Witches they need to defeat in the present day, while also ensuring the Witches can respawn. Sayaka, though, is not happy about having to do even more work when she's so exhausted already!
Still, what's done and done, and she does travel back in time to close the loop. She returns home safe and sound - but with a leaf accidentally stuck to her. Still, Mifuyu and the PMHQ, all gathered before the mirror portal to the past, figure it shouldn't harm anything, and agree to shatter the mirror. When they do so, however, to everyone's shock, the leaf abruptly vanishes from existence.
Iroha relays this story to Touka, which puts Touka in deep contemplation. After much thinking, conclusions are thus: Parallel timelines and time paradoxes can both occur via Endless Mirrors. The Mirror Witch seems to serve as a sort of corrective function for time paradoxes that occur through her labyrinth - for example, since the leaf can no longer return to its original timeline, and it was not destined to travel to the current one, it was instead annihilated. It also raises the alarming possibility that the Mirror Witch is seeking out connections to parallel universes, with the goal of finding the ideal one for her to migrate to - which, in Touka's estimation, would destroy their own timeline. Even so, all of that remains speculation - and with the Mirror Witch herself being hard to find and dangerously hard to defeat, all they can really do is monitor the situation and break the time portal mirrors as they encounter them.
Finally, just as their conversation wraps up, Iroha and Touka realize that with the mirror broken, everyone's memory of the Witch invasion is being corrected, too, into a sort of collective waking dream instead of reality - an event that had truly existed, yet no longer does.