The Quixotic Campaign: The PCs are the servants of an older, powerful NPC (both in terms of personal abilities and politically) who has gone a bit senile.
As a result of his mental infirmity, he is often much less competent than he used to be. His combat skills, if he had any, are greatly deteriorated. If he is capable of spellcasting or manifesting psionics, he memorizes or casts incorrect/radically suboptimal spells and powers. And he gets worse after sundown.
He does, however, show occasional flashes of his past competence. Some of his political speeches are simply brilliant. If he's a martial NPC, there are days when he fights like he did in his 20's. If he's a spellcaster or psionic manifester, he can be roused to use his powers to great effect. But those days are becoming rarer and rarer...
Still, he is such a great man of such great accomplishments that most people don't realize he's deteriorated (or how much) unless they spend a lot of time around him. As a result, he has lost none of his pull in society, and is still tapped for important missions for the kingdom. Negotiate a treaty. Kill a fearsome beast. Explore a new overland traderoute. Deliver a diplomatic package.
Worst of all, he's not always aware of his disability. He can become angry with those around him and lash out- sometimes quite dangerously- at his servants. Fortunately, his anger passes as quickly as a summer storm, and only remembers the slights of his actual foes.
The PC's mission- however they justify it with their particular backgrounds- is to maintain the NPC's reputation by assisting him in those missions he chooses to accept. This means they'll have to be the mind behind his voice, the sword and shield before his will.
They have to keep him alive. They have to slay the dragons. They must blaze the trails. They must find the leverage that will make the treaties work.
If they succeed, their power will wax as his wanes, and eventually, they will inherit his estate.