Agent 347 was a male Human whose ancestors were abducted from Earth in the 4th millennium BC for a eugenics project intended to secretly shepherd Humanity through its early technological age.
With his female partner Agent 201, 347 was sent to Earth just prior to 1968 to meet the threat posed by the militarization of space and the nuclear arms race.
The two agents had the use of the advanced alien technology of their far-off hidden base world. Their headquarters was hidden in Apt 12-B, 811 East 68th Street in New York City's Upper East Side. They hired Roberta Lincoln as a secretary, telling her that she would be helping their "research for a new encyclopedia".
347 and 201 were both killed in an automobile accident in early 1968. When contact was lost, their supervisor transported himself the 1,000 light years to Earth and saw their mission through. (TOS: "Assignment: Earth")