Ours was thought to be a lost cause, back in the day. Thankfully, tourist groups are just as lax among the supposedly enlightened species as they were back on Earth. We've managed to sneak enough humans off of the preserve planet to expand our outside population when we need additional or replacement agents. The fact that immortality can be gotten for cheap handles everything else.
Working on planets from the lower strata of interstellar society is strange to say the least. They have an Earth-like quality, though they have far less surveillance, since they actually managed to realize that it's impractical on the large scale. This works for us: Not only are the surroundings of a technology level with which we're familiar, but we can work without being detected, so that the Occupants, as we've come to call them, won't know that any of us have gotten loose.
We've all memorized the Five Century Plan. The duration is the worst part, but we have to make sure that it goes right the first time; the Occupants will make sure that we won't get a second chance, one way or the other. As it happens, it's more like four different plans, each a fail-safe against the others. I kind of hope that we get to the last part, though: Normally, I'd be opposed to genocide, but it'd be an eye for an eye, and they did take Earth from us, after all.