Well, I suppose I should...

As for the topic, this has been discussed several times...
Like
here, and
here, and
here, and... well, you get the idea. You're new, so I won't get mad or anything, but it is advisable to use the "Search" button.
Anyway, welcome to the boards! If you're looking into getting into user-made campaigns, you might also want to check out the Freespace Campaigns forum a little further down.
As for the story, there isn't really a set campaign that specifically "follows the story", as none of us know what the story is. If you want good post-Capella campaigns, try (rereleased) Derelict, Inferno, Warzone, or any one of the available campaigns hosted here (scroll down even further). If you want some other good ones, try Deus Ex Machina for a good laugh (and Just Another Day, though you'll have to have Freespace_Open for that; check the SCP forums), Sync and Transcend for pure spooky atmospheres, or Homesick and Sol: A History simply for their compelling storylines.