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 use 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?

As of Betrayal at Monadhan (Chapter 14, Dungeon #170):

• 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.
• The githyanki invasion has been thwarted but Tiamat's plan continues, with a dark ritualist working deep in the Shadowfell.

What is going on in the tiers of play?

Heroic Tier

• The heroes engage a number of foes, most (all?) of whom have some connection to Sarshan, a shadar-kai arms dealer.
• Many of the locations around Elsir Vale concern some interplanar portal or other, of interest to Sarshan and his githyanki paymasters.
• A preliminary incursion by the githyanki is defeated, thanks in part to the actions of the heroes. 

Paragon Tier

• Sarshan is defeated, but it becomes clear he is not the prime mover of events. 
• It is now known that a githyanki invasion is inevitable and preparations are being made to defend against it.
• The heroes do their best to persuade and recruit various races and nations to a Coalition.
• The heroes ultimately take the war to the githyanki and deal a decisive blow in Tu'narath.
• 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.