A creature named Kyubey offers to grant Madoka a wish if she becomes a 'magical girl' and fights grotesque witches. A magical girl named Homura tries to stop this agreement, and the true nature of the contract with Kyubey is revealed.
Did You Know?
In early concept art, Kyubey had wings. See more
When Kyubey explains entropy, he says it involves the net loss of energy, which is not possible in physics. What the term actually describes is the decay of energy to its base form - which is heat - and its subsequent scattering through space, until the system reaches equilibrium, at which point all energy conversion ceases until further external energy input. See more