The party is hired to transport scrolls to a temple in the hills, far from their hometown. They arrive in town, and discover that some townsfolk have disappeared. They meet the high priest, deliver the goods, and are prepared to leave, when they find the body of the high priest somewhere in town.
It seems a small band of doppelgangers have uncovered a lead to a magic item/relic that is buried beneath the tombs under the temple. The scrolls provide information of some sort the doppelgangers need to get to the item. The missing people are being used as slaves to dig beneath the tombs (which of course are full of nasties).
The final scene should be between the head doppelganger and his cronies just as the item is unearthed.
I've kept the details out of the description, because a lot of the stuff (like what's in the scroll) can be campaign-dependent. But if the players are perceptive/paranoid, they might blow this into a full-blown campaign: Did their employer know the high priest was a doppelganger? Is there a conspiracy to get doppelgangers into power in the human world?