Real world scientists have accidentally opened a tear in the fabric of reality, transporting their laboratory into the game world. Unable to repair and/or refuel their equipment in this (to their minds) primitive world they are now trapped.
The local band of 'nasties' (goblins / orcs / whatever) lives in relative peace with the local population.
Along come the PCs and go through their usual heroic monster bashing routine, wiping out the nasties and pinching all their stuff, then continue on their way.
Problem is, they don't kill ALL he nasties. The survivors want revenge and, after spending a few months recovering, start to take it in their own inimitable style (which is not unlike that of the PCs come to think of it).
Next time the PCs are in the area they find themselves VERY unpopular with the townsfolk.
A plague has hit the local area.
In humans it affects only the most vulnerable, the children and the very old, and even than it's little more than a summer cold. 24 hours of sniffles and then it's gone, barely noticeable really.
To sheep however it is fatal and the whole economy of the area is in serious trouble. If this keeps up the area could well be facing famine.
Somebody (enter the PCs) must find the rare herbs needed to make a cure.
Coinlake sits perched between two sheer cliffs in the Stigrani range, and is difficult to find, much less approach. Four miles long and two across, the water is a vibrant cyan blue. The lake's shoreline is an unassuming beach of gray pebbles, and its mean depth is seventy feet. The rare times when the sun makes its way between the cliffs and shines over the the still water, one could see clearly the lake's rock-strewn bottom.
Strangely, no fish or aquatic life can be found here.
At night, a peculiar phenomena occurs. When the night sky is clear, the moon and stars are reflected in the lake's surface, but if one were to look at the surface from a high vantage point, the reflection does not match the firmament above!
Instead the water's surface reflects the night sky of some other distant world and seventeen shining golden moons besides, each ones shimmering upon the water like so many gold coins!
Legends whisper that Coinlake is not a lake at all, but a gate or nexus, to some distant alien world.
The mystery has long remained unsolved, and only recently has the Arch-Duke commissioned an expedition to uncover the secret of Coinlake once and for all. Among the team members are several scholars of the Nascent Academy, an astrologer from the Occultists Guild, and of course the PCs, acting as body guards.