
First, we will begin with the overall story line. Starting with the broadest of plots we look towards the official Diablo III site. On the main page it states:
Official Blizzard Quote:

When Deckard Cain returns to the ruins of Tristram Cathedral seeking clues to defeat new stirrings of evil, a fiery harbinger of doom falls from the heavens, striking the very ground where Diablo once entered the world. This fire from the sky reawakens ancient evils and calls the heroes of Sanctuary to defend the mortal world against the rising powers of the Burning Hells once again.
This sets up the basis to the story that we will be going through in

Beyond this mysterious omen, we also know that we will begin the story with Deckard Cain where we find him in



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Official Blizzard Quote:

It has a name. It is not Salvage Cube. The name does include Cube, though. The name is sort of a spoiler, which is why we haven't revealed it yet.
Now that we know how the game will begin, we need to know where the plot will go. The biggest clue we have of the overall storyline is a quote from Jay Wilson in an interview with IGN during the 2009 Blizzcon.
Official Blizzard Quote:

Well, our story tends to focus a lot more on the two major demons and the impending invasion of Sanctuary.

Supposed images of Belial and Azmodan
While he never directly states who these two major demons are, there is some educated thought that can help us figure out who he is talking about. For those who do not know much about the lore of Sanctuary, there are seven greater evils. These evils are made up of three prime evils: 






Official Blizzard Quote:

We have this conundrum early on of should you have a game called Diablo without Diablo in it. We can debate that, whether we should or shouldn't, but what it comes done to is we've had a storyline in mind that's been going back quite a ways. So, without revealing too much I can say that there's been a plan. If the player thinks they've outsmarted forces of Hell and evil and they basically won, well, they are in for a big shock. Because really what they are going to discover in Diablo III is things pretty much went exactly the way Hell wanted to all along.
Moving on to the climax of the story, how it all ends is what is the most important part of the story. However, because of this, we know the least amount of information concerning this topic. As cliché as it may be, the game may very well conclude, once again, with us defeating Diablo. Perhaps his exile from Hell was part of his master plan to be reincarnated into the form we see him in now. Many wondered if Diablo would even be returning in this third sequel. However, it has been officially confirmed that the demon seen at the end of the cinematic is Diablo and that he will be returning in Diablo III. Many people will still argue that he will not actual be a demon you fight or that you will not even face him in the original Diablo III but he will not show up until one of the multiple expansions they have planned for the game. Regardless of when and how he appears, it only seems right that the Lord of Terror is, at some point, the climax to the game. After all, we know that Diablo III and its expansions will conclude this story that started fifteen years ago with Diablo I.
Official Blizzard Quote:

We're not saying this is the end of the Diablo universe, but we are trying to bring this storyline to a close. It's not just Diablo III -- we've got plans beyond
Now with the all of the story we know so far explained, we need to know how we will actually play out this epic journey. Blizzard is attempting to expand the universe in Sanctuary and there are 1,000 places we could end up at along this path. While each act will span a larger area than before, there will still be distinct areas in which each take place. We also know we have seen the first three acts that will be in Diablo III.
Official Blizzard Quote:

You've actually seen some of the first three Acts already, and we probably won't be revealing too much more than what we've already shown. We really want to try to keep as much a surprise for you when you play through the game the first time as possible. We will be announcing all of the game systems though before beta, so there are a few things left as far as reveals go. While beta will obviously be a lot of fun, should you get the chance to get in and help test, it'll only be a small part of the whole game. Some people want to know everything, but we're really trying to preserve the surprise and wonderment (that we hope will come with exploring Diablo III) as much as possible.
Act 1:
We have seen quite a bit of act 1. This act will begin in


Act 2:
Act 2 will take our characters to the East venturing in the deserts of


Official Blizzard Quote:

Act two's a little bit more of an intrigue and deception-focused story, because Belial is in there.
Act 3:
This next act has us going back to the West to Bastion's Keep. Bastion's Keep was seen in the 2010 Blizzcon panel "Crafting Sanctuary". This Keep happens to be placed right on the edge of Arreat Crater where the Worldstone once stood until Tyrael destroyed it. All we know about the area is that is the last line of defense against a horde of demons and monsters. We saw a glimpse of this assault in the cinematic as a demonic army marched. It is possible that Azmodan is commanding this army in attempt to take over Sanctuary.
Act 4:
Practically nothing is known about the last act of the game. All we do know is that it will be the shortest of all of the acts leading up to the climax of the game. There is a chance that we will, once again, be venturing to Hell to defeat Diablo much like we have in the previous games. We could be doing this by venturing into Arreat crater.
There are a few missing holes in this act progression compared to the story. There has been a lot of concept art of Jungle-type areas that do not seem to fit into any of these acts as far as we know. This could be explained by the unknown transition of all of the acts and just how we will be going from one area to the next. Even in between acts 2 and 3, we seem to be going from one side of Sanctuary to the other. There has to be some story explanation that has us traversing such a large area of Sanctuary.
Now, with the story spelled out and how it translates to the gameplay, we are left with a few questions about Diablo III. The first is what is the harbinger of doom from Heaven? Why are Belial and Azmodan trying to take Sanctuary now that they have control over Hell? Do the angels care about what is going on in Sanctuary between the humans and demons. Most importantly, how will it all end?
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HOW DARE YOU DOUBT SCYBERDRAGONS UNIMAGINEABLE POWER!!!!
SMITE HIM SCYBER!!!
Of course I could be completely, absolutely wrong, as I also don't have any legitimate guesses at what A4 will be.
And oh noes! Dun hurt meeeeee
Not all of what he said may be top-shelf material, but I thought the "tester leak" seemed to have solid material on the plot - tyrael, the player binding himself to maintain the barrier between the worlds at the end, etc.
SORTA KINDA SPOILER