Each Campaign of Guild Wars has its own PvE storyline. The stories of the first two chapters take place almost concurrently. Players traveling from Tyria or Elona to Cantha will jump into the story about 20% of the way, and players traveling from Cantha or Elona to Tyria will jump in about 30% of the way into that story, and players traveling from Tyria or Cantha to Elona will jump in about 10% of the way into the story.
Here are the storylines of each campaign: