Planet: Organia. On first appearance, a simple agrarian humanoid race that had no significant advancements for over 10,000 years. On stardate 3198.4, the true nature of the Organians revealed an extremely advanced civilizations of noncorporeal beings, having developed beyond their physical bodies a millennia ago, and who merely created the physical appearance of a simplistic culture for the benefit of the corporeal life-forms that visit. The Organians are responsible for the Organian Peace Treaty in 2267, which is responsible for peace between the Federation and the Klingon Empire until the two allied many years later.
A century earlier, in 2154, two Organian entities had an encounter with the Enterprise NX-01. They occupied the bodies of crewmembers in order to observe the humans' response to a deadly virus on a planet they had just visited. In defiance of their own protocols, the Organians ultimately intervened to cure the infected crewmembers, then erased the memories of the encounter among the humans. At that time, they decided to recommend preparations for an official first contact mission — though they thought that at the rate humans were progressing, they'd have 5000 years to prepare.