Before humans knew that beings existed outside of their own world, before, in fact, humans had gone much further than sending a couple of people to their local moon, they visited. They'd always managed to remain hidden, but their work was kinda starting to get noticed. They'd find those who were sick and injured, and amongst the human there were many. Mostly, they worked with kids, however. At night, when the entire area was asleep, they would take the children aboard their ship and treat their illnesses and injuries, then, before dawn, send them safely back to their homes and their beds. Problem is, someone's always gonna notice when a kid who seemed to be at death's door due to conditions like cancer, is suddenly completely healed.