It's a series about magical girls. You're supposed to go into it without being spoiled, I guess, so...
...I didn't, and it was still pretty damned depressing. Kyubei, his heart burning with utter neutrality, offers to grant girls a wish in return for turning them into warriors for justice for the rest of their lives, which are likely to be short. That's the part he tells them.
He doesn't tell them that afterwards, they'll spend an equal amount of time as one of the evil murderous witches that magic girls fight. Or that they actually die as soon as they make the contract and control their body remotely from their soul gem.
Most of the series is an utter crapsack festival of suck version of a magical girl world, as Kyubei tries and tries to get Madoka to make a contract because for some unknown reason she'd be ridiculously powerful. "And then become the wickedest of witches. If you ever want to kill yourself for the sake of the universe, call me." That's said after she (and the few survivors) find out the truth.
It turns out that in reality, the magical girl world wasn't quite as bad before Homura (the real main character, but also a barely-seen antagonist for most of the series) started messing around with time, trying to save Madoka. Over and over and over and over, things getting worse and worse each time, because every time she rewinds the clock to try again -- which she can do because her Wish was to save Madoka -- Madoka gets more and more powerful. To the point where by the iteration that most of the series takes place in, if she *does* become a magical girl the *world will be destroyed* by her as a witch.
But the ending, OH MY GOD. Madoka tries to find a third way (between letting the big evil witch that kept killing her prompting Homura's time loops destroy the city, and becoming a magical girl and destroying the world) and does. She wishes for no more witches! She becomes a Valkyrie, appearing to each magical girl just before they cross over to despair and destroying them, taking all their evil into herself, and then when she finally finishes collecting all the evil, and is about to change into a witch that is powerful enough to destroy the universe, she appears and kills *herself*. Happy ending! I mean, aside from Homura not really saving her. Other than that, right?
... yeah, no. Kyubei is still there, recruiting magical girls, who still die just as quickly, fighting the evil that is still there, but now instead of being collected into witches, it's free floating and takes more work to collect. But now everyone's okay with it because the evil isn't being *caused* by him. The magical girls aren't being *tricked*. The whole situation is *at least as bad* and Madoka sacrificed herself for *nothing* except that now the procedure better fits with human values.
Not to mention that the magical girl world that they started with, before Homura's time looping, no one knew that the witches were magical girls and everyone was just as happy to be fighting them. So Homura's suffering and Madoka's sacrifice really did accomplish nothing. They just cancelled each other out.
Ugh. Gotta love fridge horror after an already horrifically horrifying series
...I totally want to write a MLP version of it. I almost did, with Kill Twilight Sparkle. As someone noted in the comments. q:3