I think Doug has the overall outline in one.
The first adventure always has the PC's in a tavern or inn. Historically, the PC's hear about some nearby place where alot of treasure is to be had, and despite the fact that there are many NPC's of higher level in the town, none of them have at any time in the past few centuries ever decided to go claim for themselves what a band of 1st level characters can do. Lately this trope is being replaced by the PC's are in the middle of a festival when monsters attack them. Frankly, I've done both.
The first adventure usually involves the PC's finding some clue that hints at a larger conspiracy. The second adventure usually starts with the need to rescue some innocents, who have been taken as slaves/sacrifices/food by the cultists who were behind the events in adventure #1. When the PC's catch up to these evil doers, they discover the tip of the larger conspiracy hinted at in the first adventure. At this point, campaigns tend to split into one of two molds. Either they become mega-dungeon modules centered around the exploration of some multilevel hodgepodge of every possible underground setting (caves, mines, dungeons, underground city, catacombs, etc.) or else they turn into adventure paths where the PC's are thereafter chasing down the boss of each adventures end boss.
Somewhere around the mid-levels, there will be a city adventure/political intruige revolving around a 'who done it mystery' where the murderer is the boss of the previous endboss. When well done, this is often a 'Scooby Doo' plot, where the person who did it turns out to be some seemingly unimportant NPC introduced in the first adventure.
After that, the PC's will learn that the evil cultists headquarters is in some exotic location surrounded by forbidding wilderness, so the PC's head off on some wilderness slog because if they don't do it now, they'll soon be sufficiently high of a level to render travel more or less irrelevant.
And after that Doug is absolutely spot on about the reveal of the extraplanar bad guy pulling the strings and the need for the lost MacGuffin of Many Parts and how the rest of the game plays out.
The thing that I think should be emphasised is exactly how well all of this works, how adaptable it is, how easily it can be reskinned, and how long it takes to get tired of the basic concept. To a novice DM, I'd absolutely suggest going with some paint-by-numbers plot like this as his starting campaign to learn the ropes before trying something more novel. Heck, I've been doing this for 25 years, and my latest campaign boiled down to its essentials looks alot like the above summary.
Eventually, they fight the four servants, recover the orb, and at last prepare to face their adversary.
