The Plan is an elaborate attempt to undermine the nature of reality, started by a Discordian sect founded by Spa Insun in 1776 to help the Freemasons found America.
Wait, that's not right.
The Plan is a cross-thread, forum metagame, started by Spain Sun in an attempt to alleviate boredom.
It has no goal, purpose, or aim, other than to be random, confusing, strange, and above all, fun.
The Plan currently resides in the thread 9+9=22 (ITT: Mindscrew), but has forayed into Be Your Avatar and We Are Our Avatars, current schematics paint future forays as possible, but not a given.
The Plan began to be implemented when an unspecified number of people broke out of a Facility in the middle of the desert in 9+9=22, how this was done and why is not known. Members of The Plan are said to be "higher up" and "farther out" than characters in other Forum Games, what this might mean is unknown. Spain Sun has sent her minions into other threads on two separate occasions.
Tropes That Apply to The Plan
Arc Number - 9+9=22