Doing an extended storyline approach requires a little planning beforehand.
1. Identify a central theme of what the campaign will be about. Think about how many levels you want it to run before you continue the campaign with a new story or start a new campaign. Let's choose something simple like, "An evil temple wants to establish itself as the main religion in the country." We decide that this will last 1st through 10th level.
2. Next break up the central theme by how many parts it consists of. This should give you a good range of levels to work with. If you break up into 10 parts, then each part should allow the PC's to level up 1 level. If you break it up into 3 parts, then each part should get the PC's to level 4 by the end of Part 1, level 7 by the end of Part 2, and level 10 by the end game part. Note, the more parts you state to run the campaign, the smaller each adventure; however that involves more detail and planning at the beginning. The less parts you state to run the campaign, the longer your adventures will be but will require "filler" material to keep the players engage. Let's just choose three parts so now we have three adventures that we'll need to come up with.
3. The first part should be for characters levels 1 -3 with them leveling up to 4th level at the conclusion of the game. So what does the evil temple need to do in order to accomplish it's task? There are lots of things:
a. Forge alliances with the powerful through extortion or promise of power/profit
b. Create a scandal/disparage with the current main religion
c. Look for a magical artifact to increase it's magical and physical power.
d. Secretly back a war in which it will step in to offer "aid" in order to come across as heroes.
e. Perform a very powerful ritual that will unleash a demon/devil that can destroy the country or bow before the evil temple's power.
f. And so on...keep adding ideas here and then pick a few.
For the first part, let's choose secretly back a war. Undead floods down from the mountains attacking the countryside. The country is heavily defending itself, but this evil religion steps in and offers aid. Normally, the country would rely on the good clergy, but they are far outnumbered to deal with the undead, so the country reluctantly accepts. The PC's role is that they are sent up in the mountains to abandoned cemetery to clear out any undead there. Since we have three levels of adventuring for the first part, we need to:
1. Investigate a village which has a suspected necromancer and find him.
2. Get the location of the cemetery where the undead are coming from and stop the source of power that's causing them to rise again.
3. Explore the catacombs where they discover priests of the same evil religion creating and unleashing undead.
At the end of the first part, the PC's have evidence that this war is not what it seems, so who should they go to? The king of course.
In the second part, the temple is working to ensure their alliances through extortion or promises of power. The PC's, bringing evidence, that the evil temple is behind the war, are on the way to the capital. However, the evil temple is not stupid and through divination, discovers that there are others who know the truth, so they send assassins. At a stopover town, the PC's are attacked and whatever evidence they have is stolen. No one is going to believe their story without their evidence.
They track down the assassins hold up in some ruins and rumble. They recover their evidence and get to the capital. They immediately find out that there are warrants for their arrest and that the king has appointed a new priest as his divine advisor, the high priest of the temple of evil! The PC's have to hide and seek out allies in the capital who will listen. They find a noble who is not corrupted and the PC's discover that evil temple is holding the king's family for ransom with the help of a grand duke. The PC's discover this location and go out to free the royal family.
At the end of the second part, the PC's have reached 7th level and the king is seeking to arrest the high priest. The temple is on the run, but they are not finished. Now there are two ways to play this part out, the PC's discover the "hidden" temple where there is likely to be the high priest and his entourage. The king will send troops and supplies along with the PC's to take the temple. If the DM doesn't want that to happen, then have the troops attacked by temple minions and held up or slaughtered. Then it's up to the PC's. Or the PC's keep the knowledge to themselves for whatever reason and then they go deal with the temple on their own terms.
However, to get there, they have to go through a blasted wasteland in the mountains (i.e. undead and goblinoid encounters here), a slavery ring in which the temple is digging for an artifact to exact revenge upon the king, and then the final temple itself.
Tada! Extended plot campaign.

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