I have one idea I need to flush out, for a regular client who has their child picked up from private school everyday and delivered home. The catch being that the kid if a changeling and the local fey want to kidnap him.
A good way to start with this would be a classic. The characters get to the school to pick up the child, approach a teacher, because they don't see the girl there and he goes "Oh, did you forget something?" The girl has already been picked up by duplicates of the player characters. Bonus points if it pretty much just happened, so the players saw their duplicates drive away, even though they didn't see their faces. They'd still have the same clothes and car, so the players can start to chase them.
But why take the girl? The child is a summer changeling in a summer controlled city, and winter wants to push in on the territory. Usually, they wouldn't resort to something as petty as this, but a few rogue elements of winter have decided to take the matter into their own hands. Maybe they are changelings as well, so they are not 100% bound by faerie laws, and they want to impress their faerie parents.
Next, summer could attack the players, since they are the ones that have abducted the child, they have been seen. This could easily happen during the chase, when summer got the ransom pretty quick. Once that is sorted out, the bad guys got away, and the players have a deadline by summer to get the girl back. There are only a handful of winter strongholds in the area, they just need to get to the right one. They look for a being of winter to help them, find the right one and fight/lie/pay their way into the stronghold and rescue the girl.
Should be doable during one session. If you need to cut things short, you can, for example, have one of the winter changelings defect and look for the player characters to tell them where they can find the girl. Though I'd wager it feels much better if the players find their way there themselves.
