An old prospector has been jailed for a number of years over a large theft of treasure. The authorities never actually found the loot, but rumours have spread and other parties are becoming interested. As the date of the old man's release grows closer, a local gang is hanging out for him, and are planning to force the old man to lead them to his loot. However, corrupt authorities are also interested, and have planted fake prisoners in the old man's cell in order to get hints on where the treasure might be hidden.
Maybe the corrupt officials will try to strike a bargain with the old man, asking to be taken to the treasure peacefully, or they might let him loose into the more 'rough' tactics of the local gang waiting outside the jail.
Maybe frantic locals are already mining in droves in the nearby mountains, or maybe the rumours have spread enough to create a 'gold rush' for the hidden treasure.
The players could stumble across the small mining town, and get involved somehow - maybe they are interested in the treasure themselves, or maybe they would like to save the old man, hoping to become allies with a supposedly rich ally!
You never know - maybe the treasure doesn't ACTUALLY exist…