Hokay. So one time Mami convinced this chick called Michiru Kazusa (better known as Kazumi), who had terrible fashion sense and a food fetish but was otherwise basically likable, to make a contract, and trained her. Kazumi quickly outgrew the nest, and found her own group of contract-ready girls, whom she named the Pleiades Saints. The seven-part team somehow fought for a while without running out of Grief Seeds until one of their allies, a girl named Yuuri, witched out. Kazumi was heartbroken at dooming her allies and shortly followed her. The surviving six resolved to find a way to turn her back.
One of the people in the group was a girl named Kanna Hijiri, who due to guilt over a past event, had wished to create another version of herself to live out her civilian life while she fought witches as Niko Kanna. Niko was very technically-minded and had a reconstruction ability, which she used to transform a spare Kyubey body into a being known as Juubey, which she believed could cleanse Soul Gems without a Seed. Another girl, named Umika, used an extremely powerful memory magic to remove knowledge and perception of Kyubey from themselves and everyone in the city. The six of them also began using what remained of Kazumi's body to create clones, but after twelve failed attempts, with the clones becoming violent and dangerous, they opted to create a thirteenth clone without Kazumi's memories. They also started catching MGs who entered the city, taking their Soul Gems, and putting them into suspended animation.
Meanwhile, the artificial Kanna Hijiri learned about the existence of magical girls and her true nature, and turned evil because abloo bloo bloo not real. She gained a ridiculously hax ability that let her do pretty much anything that could be worded as "connection", and resolved to kill the Saints and make the entire world nothing but artificial people (termed "Hyades.") Meanwhile meanwhile, Yuuri had a friend that she wished to heal, named Airi, and Airi found out too, wishing to take on the form of her friend to kill those she believed responsible. Kanna enlisted her help or something, I dunno, this is when things start getting dumb. (Start?)
So then the clone got into a suitcase for some reason and was also naked for very obvious reasons, got involved in a tedious filler plot with a bomb, and was rescued by two of the Saints. I'd name them, but only two of them were actually important to the plot. Anyway, they found out that someone (Kanna) was creating artificial Grief Seeds called Evil Nuts that could turn humans into witchlike beings and witch out MGS outright. Then they fought a few of those witchlike beings, then they fought Airi in the form of Yuuri, and killed her after a whole lot of tedious speechifying.
So anyway then they had an arc where Kazumiclone and Niko fought the Souju sisters, two magical girls that occupied the same body and had different powers. They won, but Niko then proceeded to witch out because her Soul Gem wasn't actually being cleansed, it was being "polished" (what that means, I don't know). This caused the Pleiades to take Kazumiclone to an underground facility where they had placed all the girls they'd captured.
Then a whole bunch of other stuff happened, Kazumiclone started to turn back into a Witch and absorbed the twelve other clones, and Kanna decided to show her hand by getting into a big dumb fight and turning all the captured girls into witches. Also one of the Pleiades turned traitor because reasons and died. Also one of the Pleiades was really gay but in a creepy yandere way and died. Also one of the Pleiades was really gay but in a somewhat healthy way and got turned into a witch. Anyway, that left Kazumiclone, Umika, and Umika's soccer-playing girlfriend to defeat an army of witches under Kanna's control.
So the memory spell got lifted because Umika saw Juubey disintegrate because he'd tried to absorb too much Grief and realized what was going on. Kyubey showed up to be all "lol, toldja so," Kazumiclone went to Kyubey and said that since she was a clone she'd technically never made a contract. Kazumiclone made a contract, became a double magical girl or something with even worse fashion sense, and promptly beat up Kanna's army of witches. Kanna then proceeded to Voltron them into a not-Walpurgisnacht and did the whole "not so different" speech but Kazumiclone destroyed it with the power of friendship speeches and laser beams. Kanna then realized that all her plans had failed and killed herself rather than continue existing in this series. Kazumiclone and the other two survivors then spent the rest of their days hunting witches and grabbing Seeds, presumably until Kreimhild Gretchen was born in a maelstrom of black smoke and consumed the known galaxy.
And now you know what happened in Kazumi Magica. Please don't ask me to do that again.