According to MW Cooper, PLATO is the name of the project responsible for diplomatic relations with the aliens.
It is related to project Sigma, which is the central project that is in charge of communications with the aliens.
Project Plato was created in 1953 when through Sigma contact was first made with alien spacecraft orbiting the planet.
On Feb. 20, 1954, Project Plato secured a formal treaty (illegal under the US Constitution) with the aliens. The terms were that the aliens would provide the US government with technology. In return the US agreed to keep their presence on earth a secret, not to interfere in any way with their actions, and to allow them to abduct humans and animals. The aliens agreed to furnish MJ- 12 with a list of abductees on a periodic basis. (See: Preserve Destiny Project)
According to Val. Valerian this treaty was renewed, again within the Plato framework, on 25 April 1964, at Holloman AFB, New Mexico. It is possible that this resulted in the exchange, from 1965 until 1978, of ambassadors, under the project Crystal Knight. (See also 'Serpo').
The name, of course, refers to the Greek philosopher.
Sources: MW Cooper, Val Valerian, Dan Sherman. Reliability: uncertain.