Through a series of unfortunate events, one of the PCs and an NPC have crashed into each other (walking, driving, riding a horse, whatever) and in the crash both are rendered unconscious. When they awake, they learn that they have somehow swapped bodies. The mentality of the PC is in the NPCs body, and vice-versa. This could be a good thing or a bad thing depending on how the characters look at things. However, the NPC turns this into a bad thing for the PC. The NPC (now in the PCs body) goes on a crime spree in hopes that the body-swap is a temporary thing. Of course, when things are returned to normal (if they ever are) the PC will be on the hook for all of the evil deeds done.
It’s up to the PC and his or her friends to keep the NPC’s actions in check while they try to find a way to reverse the situation and return things back to normal.