Do not be fooled by the cute, happy characters in the Mahou Shoujo Madoka Magika promotional image. The story is dark and oppressive, and is definitely not intended for kids.
An emotionless alien race discovered that emotions are a huge source of energy, with the darkest emotions being the most powerful. These dark emotions occur in strange entities called "witches" that prey on an unsuspecting population. When a witch is destroyed, it leaves behind a "grieve seed", which contains the energy.
An alien, Kyuubee, recruits young teenage girls (which is quite disturbing all by itself) who are granted one wish in exchange for becoming a mahou shoujo (magical girl) with the power to fight the witches. The girl is given a small gem which is supposed to be the source of her power. The gem accumulates impurities which can be transferred to a grieve seed, before Kyuubee, er, "eats" it, which is how the aliens collect the energy. Don't ask; it is never explained.
The story follows Kyuubee's attempts to recruit the title character, Kaname Madoka, while a time travelling mahou shoujo, Akemi Homura, desperately tries to convince Madoka not to accept. It quickly becomes clear that Homura knows much more than she is telling.
Wishes always have unexpected consequences and in this story there are two huge ones. What exactly happens when one is changed into a magic girl? Where do the witches come from? Knowing the answers to those questions, no person would accept the deal.
Which is why Kyuubee lies by omission, unless asked a direct question. Its answers are often cruel and heartless because, as an emotionless alien, Kyuubee has no understanding of humans at all. Kyuubee may be a cuddly looking creature but it is definitely on my list of least likeable anime characters.
Given the nature of the series, a happy ending was never an option, but it was reasonably satisfying nonetheless.