Was that the one that you fight a huge version of Q in space? When you say story missions, do you mean we need to finish the cardasian / romulan episodes first?
In your journal, the top-left box is for story missions. Click More, and you'll get a full list of story missions in order. In order to unlock the Breen missions, you need to either finish all the missions before them, and meet the level requirement listed (I'm pretty sure they're in the 40's so you're good). If you don't want to do the missions before it, there's a skip button that sets them as if you completed them, you can go back and replay later or just ignore them entirely.