I'm running my slightly Harry Potter-esque campaign this Saturday, and playing in another game on Sunday.
In the Potter-esque game, the PC's (some of whom are near to graduating from a wizarding university, others who are honored guests, all 4th level) have just discovered the source of severe bad dreams that have been plaguing the students of the school for the past year. Someone has been sending small spider constructs into the school to do something to the students. The spiders travel on the Shadow Plane (which in my game serves the same purpose as the Ethereal), only shifting into the Material Plane to draw little bits of their essence out of them while they sleep - exact details unclear.
Due to the magical protections on the school's boundaries, the things have been smuggled into the school physically - they couldn't walk into the school while in Shadow, but can shift into Shadow once inside school grounds. The smuggling has been done by little creepy gnome-sized beings, several of whom burst into a magnesium flash when killed. They captured one - the Psion charmed him - and this session will see them try to discover what he is, how the spider constructs work, and what they're up to. I've got some foreshadowing of various things to toss in there, and some combats planned.