If you're writing your own D&D adventures, you are probably familiar with the 3-act plot structure. I have wondered if this is where the 3-encounter format of Dungeon Delves came from.
Recently, I came across a website describing a different type of act structure that the vast majority of successful films uses, with nine acts and two goals.
The first goal is what the protaganist thinks he needs to do. The second goal is what the protaganist discovers he needs to do. For example in Star Wars, Luke's goal is to return R2D2 to the Rebel Alliance. However, part way through the movie, his goal changes to destroying the Death Star.
This 9-act plot can be summarized as follows:
Act 0: The Backstory - stuff that happens before the adventure begins (the scrolling text at the beginning of Star Wars).
Act 1: The Beginning - the story, uh...begins. This is usually an establishing shot. (Princess Leia's ship getting attacked by a Star Destroyer).
Act 2: Something Bad Happens - the conflict is introduced. (Darth Vader captures Leia).
Act 3: Meet the Hero - the protaganists are introduced. (Hello Luke!)
Act 4: The Hero Commits - the protaganists decide what they need to do...either by choice or they're forced into the choice. (R2 escapes and Luke pursues him).
Act 5: Pursuing the False Goal - the protaganists act on what they believe they need to do to succeed at their original goal. (Ben, Luke and the droids meet up with Han and Chewie and are off to Alderaan...where they get forced to land on the Death Star. They rescue Leia).
Act 6: The Lowest Point - the act begins at the lowest point, but the realization that their original goal isn't what they need to do arrives. (Ben dies, the Falcon escapes).
Act 7: Pursuing the True Goal - the protaganists decide what they need to truly do, and then do it. (the Rebels plan on destroying the Death Star. The Trench Run. Vader almost kills Luke. Han and Chewie return just in time).
Act 8: The Denouement - loose ends are wrapped up (Leia awards medals).
So how could this act structure apply to D&D adventures?
Act Zero: The backstory is usually supplied by a combination of the campaign setting and specific elements about the villain of the adventure. This backstory should be specific to the plot, but may not even be revealed to the PCs.
Act 1 and 2: The beginning and the the Something Bad probably occur off screen as far as the PCs are concerned. The princess is captured. A caravan has gone missing. The gods have gone missing.
Act 3: As the PCs are the heroes, there obviously isn't much to this act, except of course if a new PC (or perhaps a companion character) is being introduced.
Act 4: Something happens to the PCs to get them involved. Often this is the first encounter of the adventure. They come across the ambushed caravan. Asassins attack them.
Act 5: Pursuing the False Goal. This is probably a series of encounters ideally progressively getting nastier as the PCs enter the dungeon or wilderness in pursuit of their foe or to find the Lost City of Gold.
Act 6: The Twist gets Revealed. Ideally this should be a very exciting or nasty encounter in which the PCs discover that what they're really after isn't what they thought. They discover that the Princess is actually a Succubus. The caravan was carrying components to raise an army of undead. The Lost City's gold cannot be taken from the site and the undead occupants are angry...they have to escape alive.
Act 7: The PCs pursue the new, true goal. The PCs track down and kill the Succubus. They kill the necromancer. They get out of the city with their lives.
Act 8: The Denouement. The PCs get their rewards at the end of the adventure.
Now, with so much going on this probably requires a somewhat larger adventure as opposed to a Dungeon Delve, or even a campaign (mini or full sized). Many of the campaign summaries that WotC has published in books like Underdark and the Plane below feature two-goal plots, but unfortunately my impression has been that most of the published adventures do not.