In role playing the GM has a duty of entertaining the players. That's a given, but ultimately the players are the one making the game fun or not.
The GM will provide a plot, something to launch the campaign - starting a quest.
Example of quest :
- a legendary item has been stolen, your group is asked to retrieve it
- a convoy has to go in another town and has to cross into a dangerous territory ; the group has to defend the convoy
- a recurring villain has kidnapped a princess and the group has to rescue her
- a member of the team has been poisoned, cursed, been hypnotized and the group has to find the antidote, cure, etc...
- a dragon has been spotted over a village, the group has been paid by the villagers to protect their village
- a member of the team has been kidnapped and will be forced to fight into an arena ; the rest of the group has to rescue him or her.
- basically it's all about retrieving items, protecting an area, or rescuing each other.
But the player are the best during the game to generate sub-plot, and the GM well planned campaign can really go in another direction.
The constant fight between good and evil is always the basic plot - but usually the dichotomy starts to become blurry as soon as the players are putting their agenda into it.
At some stage, it looks like there are various possibilities for the "Good" (defending the villagers, rescuing the princess, exploring the cave, etc ...), even though the good are supposed to work together and be united, usually the group get divides on what to do. Think of the Lord of the Ring, the team had to split after the Moria.
But on the Evil side, it's even more possibilities, because potentially each villain can have his own agenda, may it be the search for eternal life, for the ultimate power, for the complete domination of the world, etc ...
Which means that even if the "Good" party has been defeated, the villains will be fighting each other, using treason, treachery, assassination, lies, etc ...
From a lawful good prospective, the villain is a tumour that has to be eradicated - because once it's planted somewhere, it will put havoc and will spread.
Now humanly, the only way to fight evil is to understand its root.
The lawful good mentality will generate more wars, and dramas that will in turn prepare the next generation of villains, because the show must go on - this is tolerated by the GM ; it will supply an endless number of future campaigns.
In old school role playing the plot isn't to turn the villains into a friend, to convert him by making him realize his actions are destructive and pointless...
To prove him that he's on the path of destruction, his own destruction - because of his fear to be judge, his fear of losing control, his need of power.
What is destruction ?
When the "good guy" and the villain will eventually kill each other - when the last combat will be over.
Then something new can start over.
And that's what it's all about - this has to come to a point of the final confrontation.
Think about Ragnarok now and ponder on it for a while.
Lecture: The Work of Secret Societies in the World: This levelling-out process will bring war and bloodshed in its train — war in the shape of economic strife among nations, pressure for expansion, suppression in every form, strife in the sphere of investment and profit, industrial undertakings, and so forth. And by adopting certain measures it will increasingly be possible to handle vast masses of people by sheer force; the individual will acquire greater and greater power over certain masses of the people. For the course of evolution is leading, not towards greater democracy, but towards oligarchy of the brutal kind, in that the power of the single individual will immeasurably increase. If morals are not ennobled, this will lead to brutality in every possible form. This state of things will come, just as the great water-catastrophe came to the Atlanteans.