Basically it's a Puella Magi meguca meduka (Madoka Magica) AU where incubators are actually really weak and show no other powers other than to grant any wish and turn people into magical girls (then devour their souls) And basically there are only a few left because of this, and the incubators are trying to solve this problem so each city in the world has at least one incubator (the one shown here is London's incubator)