When night falls, we find peace in the knowledge that daytime will return soon, and vice versa. But what if the world was split: daytime and night are two different, coexistent worlds, each with it's own laws and rules. At dawn and sunset, our soul switches to inhabit our other being, our 'twin'.
It is possible to buy magical abilities. The process is very dangerous though, involving chemicals, rituals and surgery possibly leaving the aspiring mage insane, retarded or scarred for life.