So here's a rough outline of the premise:
- There's a fairytale world, and the real world. They exist side-by-side as alternate dimensions. There used to be a lot of travel between the two worlds, with fairies meddling in the affairs of humans and humans wandering accidentally into the fairytale world on the way to their grandmother's house. At some point, the Fairy Queen decided that all of this contact was bad for fairykind, so she established a barrier between the two worlds to prevent cross-contamination. Most humans couldn't find their way across anyway. Fairies still could, but doing so was grounds for severe punishment. Even so, the barrier has a dramatic effect on fairies crossing over: It turns them human.
- Fairy tales are true stories, or at least have a kernel of truth. Centuries of these stories being told over and over has imbued them, and the fairy world, with increasing magical power. Certain artifacts from these stories that are still around are saturated in this power -- in the world of man. In the fairy world, they're just ordinary objects: a red cap, a silver pitcher, a pair of boots, a key, a book, and so on. Take them to the world of man, though, and they're suddenly supernaturally powerful. That key makes any door open onto a secret room in Bluebeard's castle. That red cap? Whoever wears it can find any location without getting lost. Dr. Knowall's ABC Book contains any piece of information that's currently known by a living person. And so on.
- The fairy world is still accessible from the world of man, and vice-versa, through certain points of commonality -- rings of mushrooms, stone circles, strange doors, dead-end alleyways, etc. These paths are all but unknown to mankind, but fairies know of them. Each of these paths only works once per year.
- Perhaps because it's forbidden, more than a few fairies are drawn to the world of man, even though it means being human while they're there. To compensate for their lack of magic there, they're often known to take a fairytale souvenir with them. Consequently, they frequently become highly successful, thanks to the power of the souvenir.
- Naturally, this doesn't sit well with Queen Mab. These fairies are to be tracked down and punished, and their souvenirs confiscated. That's where the PCs come in: They're agents of the queen, tasked with going to the world of man (on official business), recovering these souvenirs, and capturing the rogue fairies.
- Most of the action takes place in the world of man, but it's conceivable that stuff can happen in the fairy world, too.