I feel stupid, because I didn't work out what had actually happened until I started reading the comments. I just thought that they were leaving Atlantis (I read "Who's Left and Who's Leaving" a couple of days ago, which I think influenced my interpretation of this one).

And then I read it again, and it was just as lovely this time, only several times sadder! But bittersweetly sweet, if that makes any sense. Loved it :)