As the campaign unfolds in the published adventures, it will be useful to write and develop an overview of what is going on.  Those of us who have been reading ahead will already have seen how the later adventures expand and clarify the events of the earlier ones (part of the raison d'être for this wiki, of course), and a general, as opposed to a specific, overview can only be helpful.

What is the overarching story?

Betrayal at Monadhan (Chapter 14, Dungeon #170) summarizes the plot:

• Tiamat is executing a plot to destroy her arch-nemesis Bahamut once and for all.
• In order to do this, she must perform a ritual that will summon Bahamut in his mortal form.
• She needed to create a distraction to allow her the time and liberty to do this, and engineering a githyanki invasion of the world was it. 

What is going on in the tiers of play?

Heroic Tier

• The heroes engage a number of foes whose activities are being coordinated by Sarshan, a shadar-kai arms dealer who is the main enemy of the heroic tier.
• Many of the locations around Elsir Vale concern some interplanar portal or other, of interest to Sarshan and his githyanki paymasters.
• The githyanki make a preliminary invasion, and are routed.
• Sarshan is defeated (in the first paragon adventure), but it becomes clear there is a power behind him.  

Paragon Tier

• The heroes recruit various races and nations to a Coalition to defend against a githyanki invasion of the Elsir Vale.
• The heroes ultimately take the war to the Astral Sea and deal a decisive blow in the githyanki capital, causing the withdrawal of the githyanki from their alliance with Tiamat.
• The party learn that the ultimate enemy behind the war is Tiamat, the god of greed, wealth, and envy. 

Epic Tier

• The heroes embark on the end-game to thwart Tiamat's dark plan.
• Their first step is to disrupt the ritual being performed deep in the Shadowfell.