Exploiting insecurities and prejudices of the heads of the household they squeeze out those already there.
Things escalate until a tragic denouement.
A very 'grey' film, neither the Parks or the Kims are evil, they are both exercising what power & skills they have to make their lives better in ways that they understand. Both hurt people with their actions but are very very polite while doing so. This slips as the film proceeds, especially as the layers of damage are uncovered. Both treat the other as objects and will step over them to get what they want. Is it the great anti-inequality polemic it's supposed to be? Maybe. Mostly it humanises everybody involved, at both ends of the spectrum.
As the must-see film of the last couple of years, at the cinema I was mildly disappointed. Now with a re-watch I think I like it more but it's still not quite the masterpiece its reputation would suggest because it feels like something you watch once, maybe twice.
That's all in the lap of the reputation though, it is still very very good. If you haven't seen it, it's highly recommended 9/10.