What you've posted doesn't constitute a "plot." It's barely a premise. And it's also awful. Devising an idea for a story involves thinking that idea through. You obviously haven't. Where to even begin with this ridiculousness...
In the "near future"? It's highly unlikely that half of the world is going to successfully enslave the other half anytime in the next few years. Wouldn't you agree? For Christ's sake, imagine if human beings were banned from owning pets for a hundred years starting tomorrow... What would it be like to walk into somebody's house and see a Great Dane sitting on the sofa? If you hadn't seen a dog in 50 years you'd likely be completely taken aback. You can't set a story in a world where there hasn't been enough time for people to become accustomed to the way things are. If you were to try to enslave me tomorrow, neither of us would be having a good day. I don't have any interest in being anybody's slave and anyone stupid enough to try to enslave me is going to have to contend with the fact that I'll do whatever I possibly can to remain free. But if you had enslaved my great-great-great grandfather and my family had been living as slaves for 150 years, then you'd probably have an easier time dealing with me because I wouldn't know any other life. I'm assuming that never crossed your mind.
What would be the purpose of enslaving half the population anyway? What sort of work needs to be done? About ten percent of the population of your average developed society is engaged in agriculture. Manufacturing employs less than ten percent of the population. So if you've got the most difficult jobs being covered by less than one-fifth of the population, why the Hell would you need slaves anyway? Are you going to force the slaves to do work that can be done by a machine much more quickly and efficiently? Are you going to get rid of paid employees and replace them with slaves? How do you think that might work out?
"Oh, hey Andy. Are you here to have your car washed? I've got some good news - we got rid of the machine that you drive through and now I have this fleet of ten slaves who will take these sponges in these buckets here and they'll wash your car for you by hand. I'm saving a lot on electricity so I've halved my prices. I'll just be in the office. Here - if they start slacking off you can whip them with this cat o' nine tails."
How do you think the general public would react to the announcement that slavery has been legalised? You think most people would say "Well it's about time!"? People would not be thinking "Finally!" They'd be thinking "What the feck is going on? This is completely mad." How are you going to explain how half of the population just woke up one day and decided to enslave the other half? What compelled them to do it? And why aren't most of them dead? Most people today are so lazy and selfish and spoiled that they'd take death over being somebody's slave. I'd rather be dead than wash dishes for twelve hours every day, never mind doing something more difficult or more dangerous. I'm pretty confident that most educated people from developed societies would be difficult to whip into shape to make them good slaves.
If the government is so desperate for soldiers that they need to institute a draft, then that means that people aren't lining up to join the fight. That means that the war isn't popular with the public. Who's doing the fighting anyway? Is it slaves versus masters? How would slaves be in any position to organise the prosecution of a goddamned war? Is it being fought by people who support and oppose slavery? If so, the government must have chosen a side. That would mean that it would be a civil war because a sizable portion of the public disagrees with government policy. If they're willing to go to war over it, then they must feel pretty strongly about it. And if it's the government that instituted the draft, that probably means that the number of people who oppose slavery dwarfs the number of people who support it, hence the government's need to round up more soldiers.
This idea is completely retarded.