I picked out the ones I think are key -- but those of you who are DS9 peeps, chime in! She's not spoilerphobic (I don't think -- are you?), so you can discuss plot happenings in here. And FYI, she doesn't like Jadzia, so I avoided Jadzia-heavy episodes... but you can mention those if they're your favorites for my sake.
Google reveals this website for a list of episode titles. Clicking the titles seems to bring up the synopses for the seasson.
Off the top of my head, I think you might want to see the following episodes (with a few "oh, and this happened in this episode" fill-ins from your loving friends like me in between). Ones you really MUST see because they can't just be explained are in bold:
- "Emissary" - Series premiere. WORMHOLE ALIENS = AWESOME.
- "In the Hands of the Prophets" - establishes some important secondary characters & political situation on Bajor
- "The Homecoming" / "The Circle" / "The Seige" -- the 3-part arc at the beginning of the season is... well, you could probably skip it, but I love Bajor and it's some crazy Bajor action, and the first hinting of the awesome way that DS9 does multi-part arcs. :) So you don't need to watch these for future plottiness, but they're pretty rad and forward a lot of politicking & Vedek Winn.
- "The Maquis Part I & II" - This episode is... eh, these two aren't some of my omgfavorites, but they DO set up a lot of stuff that happens in seasons 4 + 5, so you can watch them if you feel like it. Unless you've seen Voyager, and feel like you've got a pretty good handle on who the Maquis are.
- "Crossover" - OMG. WATCH EET. MIRROR UNIVERSE IS WIN.
- "The Collaborator" - if you decide you like the Bajor Arc, this is a good one to watch. Otherwise, we can fill you in. I actually didn't see this one until a few years ago, and was all "OH MY GOD, THIS MAKES SENSE NOW!"
- "The Jem'Hadar" - The first real Dominion episode. They're mentioned a bunch earlier in the season in a "who are they anyway?" sort of way, but this is a good one to see.
- "The Search, Part I & II" - DOMINIONNNNN!
- "Past Tense, Part I & II" - Not necessary for plot at all, but this was my favorite DS9 episode for YEARS, because I <3 time travel.
- "Life Support" - if you decide you like Bareil or Winn, watch this one. Otherwise we can fill you in.
- "Heart of Stone" - liminalliz will want you to watch this one, because Odo & Kira are stuck on a planet.
- "Through the Looking Glass" - FUN FUN FUN.
- "Improbable Cause" & "The Die is Cast" - important Cardassia and Dominion plotty things happen here.
- "The Adversary" - season finale, WATCH WATCH WATCH.