The easiest description of the whole magical girl genre is to just think of it as being Sailor Moon. That hits almost all of the major tropes right off the bat and most of the shows are very formulaic. Just think of it as a giant checklist.
- Cute girls ranging in age anywhere from pre-teen to highschool students. There should be one 'leader' but plenty of other girls filling in the various personality subtypes. Sweet, brainy, aggressive, etc.
- There will be a magical anime/mascott of some sort which will introduce the girls to their magical heritage as well as serve as a mentor/guide to their new life as a mahou shojou (magical girl).
- Each girl will receive an item of power which, often along with some activation phrase, will initiate a transformation sequence which turns them into a magical girl. This transformation sequence will come with a new cutesy outfit along with a weapon of some sort all of which will fit a certain theme or be color coded. Fire, ice, ranged, bombs, etc.
- Each week, the evil dark overlord will either send monsters to attack the city or corrupt/endanger normal human beings and it will be the mahou shojou's job to defeat the nefarious forces of evil and save mankind once again.
- Often times there will be a pokemon-esque Gotta Catch 'em All feature where by defeating monsters the mahou shojou can pick up some sort of item which leads to new powers or fulfills some sort of greater goal.
These shows are almost always happy, pink, and appeal mostly to young girls (and sometimes creepy older men). All of that happy-go-lucky sentiment goes out the window when you start deconstructing the genre however. It doesn't take very much effort to scratch the surface on the idea of magical girls and find something that's pretty disturbing. If you want a classic look at deconstructed magical girls, I suggest taking a peek at Sailor Nothing. It's a web-fic about how something like Sailor Moon could go dreadfully wrong. It's not too hard to come up with some of the negatives yourself. Imagine school age girls who are sent out to fight monsters each week. Just imagine the fear, not to mention the possibility of being maimed or killed in such battles. There's also the fact that other than a few friends who already know about it, there's no one you could tell about the situation or who could help you if you needed it. Lastly, consider the fact that there's some group or force out there who believes that weaponizing young girls is somehow the best way to save the world or otherwise accomplish their goals. All of a sudden your allies seem less dependable in retrospect. Anyway, that's basically the whole premise of Puella Madoka Magica.
A white cat-like creature appears one day and wants you to make a contract with him/her/it. It tells you that it's name is Kyubey and that it will grant you any one wish you care to make. In return, you will become a mahou shojou, gaining magical powers with which you will battle witches. The witches prey on mankind, stealing people's souls which causes them to become suicidal and self-destructive. By destroying the witches you not only end up protecting the people around you who would otherwise become witch bait, but destroyed witches also leave behind grief seeds, which you can use to replenish your magical powers. Sounds spiffy, right? Well, maybe not so much.
The show basically begins in classic mahou shojou fashion. Everything is cutesy and happy and the main characters (Madoka and Sayaka) hesitate in making a contract not out of any hard reservations but mostly because they don't know what they would wish for. Madoka even suggests that her one wish might actually be to become a magical girl, someone who is important and fights evil, and thus she wouldn't even need an actual wish in return. This sentiment is only strengthened when they meet an actual mahou shojou, Mimi, who on top of being sweet and sophisticated, offers to take them along on her battles against the witches just so they can see what it's like. Mimi is confident and more than competent, blasting the first witch she faces with flink-lock muskets she summons with her magic. Things go so well that it's only a few days later when Madoka tells her that she's decided to make the contract and will finalize things just as soon as Mimi defeats the current witch.
After we wrap this one up we'll go find Kyubee, make the contract, and celebrate with some cake. Hurry up, Mimi. Just blast the thing and we'll go celebrate.
*Cue expressions of horror all around*
Needless to say, mahou shojou aren't supposed to lose, much less get their heads chomped off by monsters. All of a sudden, making that contract doesn't sound like the greatest idea in the world. All I can say is that from this point in, it gets worse. New mahou shojou appear and dark truths are slowly uncovered. It turns out that in exchange for having their wish granted, the mahou shojou are, in effect, selling their souls. The process to create a mahou shojou rips the soul out of their bodies and places it inside a Soul Gem which powers their magic. Use of the magic slowly corrupts the gem and the only way to purify it is to destroy enough witches, forcing the corruption out and into the Grief Seeds recovered. If your soul corrupts too quickly or if you fail to cleanse it fast enough, the mahou shojou becomes a witch. It's like the great circle of life. Mahou shojou kill witches and one day will themselves become a witch for new mahou shojou to kill.
At this point, you have to figure that only a crazy person would sign on to this sort of faustian deal. Certainly, Madoka wants to have no part of it after seeing Mimi's bloody death but circumstances conspire against her. Her best friend has signed up before discovering the fine print and is now rocketing toward self-destruction. On top of that, it seems that the witch equivalent of Hell Night is coming to town in scant days and there aren't enough mahou shojou around to stop it. If madoka doesn't make a contract, the people in the town which includes her family are doomed and if she does, well, we all know where that leads.
All of a sudden, Kyubey's deal doesn't seem all that stellar.
Of course, I've left out a lot of bits for simplicity's sake. There's also a time traveling mahou shojou trying to protect madoka, another mahou shojou with an eating disorder, and some crazy theory about the great circle of life being the cure to the universe's entropy curse and eventual heat death. It's a hodge podge for sure. Then again, there are still 3 episodes left that haven't aired yet and who knows how it'll all turn out. It's been pretty spiffy so far.