Sisko and crew must deal with an invasion from the Gamma Quadrant of an alien species that seemingly can dissolve matter. He appeals to the wormhole aliens for help.

Once again, incredible writing, so that I could hear the characters out loud!  Great mystery, too.  It was like watching a GOOD episode of DS9.  I especially liked the way the wormhole aliens made it actual fact that "we" had devastated them millennia ago, because Sisko did actually send their enemies back in time, and they had to flee!