Who were they enslaved by . . . . ?

In "Alien Nation: Dark Horizon," the "Alien Nation" overseer sent to retrieve them told "Cathy" ( or whatever her name was ) that if she went with him, she would be with him.

But, so what ?

If he is just a glorified slave, how secure would her life have been ? How could he guarantee her that his masters wouldn't take her from him ? How could he trust them ?

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Plus, on the ship he was to return to, I remember seeing nothing but other "New Comers." This really makes me wonder who were the "New Comers" on Earth enslaved by ?

If they were enslaved by others within their own race, this looks more like a type of caste system than a racial slave system. What do you think ?

What you say is true for the most part, but there are people who love having a little power. If you've never had a mean, vindictive, sadistic boss (and bosses always have other bosses), then you're lucky. It's my experience that type of person tends to gravitate towards the type of position the overseaers were in.

For example: in "Stranger in a Strange Land", Michael learns about human society by watching monkeys in a cage. A big monkey beats up a smaller monkey and steals his food, so the smaller monkey goes over and beats up an even SMALLER monkey and takes his.

I've always found that to be a perfect example of most of human society, but then, I'm a bitter old fogey...

(Because I SAID so, that's why!)

Imagine a scenario where an Earth ship full of Nazis with Jewish prisoners crashed on some other planet. Would the people there be able to tell which ones were Nazis if they were all trying to blend in as prisoners? We can't even tell each other apart. Look how long it's taken us to round up the war criminals. We're still looking for some of them even now.

Now if we're talking white masters and black slaves, telling those groups apart depends on whether or not another species would see human skin color as significant. We might all look alike to them.

Anyway, my point here is that we have enslaved and imprisoned each other over minor things (religion, skin color, etc.), so it shouldn't be a surprising that another species might do the same. My vote goes to the Overseers being some kind of evil ruling class. Because as you say, they don't act like slaves who are fortunate enough to be beaten less frequently.