Three million refugees from a race called the Skrreeans come through the wormhole looking for their destined homeworld. It's later revealed that Bajor is the world they have sought, and the problem becomes finding a place for them on a torn world that can barely sustain itself.
However, before this premise is revealed the story spends its first act mired in a lot of silliness involving the Universal Translator, which ultimately has little dramatic relevance and winds up being simply implausible. (And just because there's a language barrier doesn't mean the Skrreeans have to enter every room with such overstated trepidation.) Once we're through the awkwardly played scene where the Skrreeans become linguistically comprehensible, a social allegory of sorts begins to take form. About all I can say here is that I see what they were going for, but the execution leaves much to be desired. The idea of the Bajorans denying the Skrreeans the sanctuary they request is definitely a plausible argument, and the issues of xenophobia and unwelcomed immigration are certainly relevant.
But the script is heavy-handed in its obvious messages, and the dramatic tension is forced. Even though Sisko finds the Skrreeans a perfectly reasonable (probably more reasonable) alternative planet, Haneek (Deborah May), the Skrreean who represents the refugees, remains positively adamant on Bajor for reasons that are never made clear. The episode seems to be preaching about Bajor's unwillingness to help outsiders for the sake of preaching, rather than building a solid story around the premise.
I also don't care for the completely forced and manipulative ending sequence, where the story sends Haneek's son Tumak (Andrew Koenig) charging toward Bajor in a stolen ship, for the sole reason of killing him off and driving home the would-be message. Yes, there are some valid points here. But there's also a lot of lackluster drama that wants to mean more than it really does.