I have always found the wiki kind of confusing, although it has been immenselyhelpful. What's the difference between prologue and old plot? I started playing before act two was released and I want to start over but I'm not sure how the chapters are different or move from one into the next.
Old plot is the original story that is not finished and it won't be updated in future, new plot is the updated story based on old plot and it will be continued in coming releases. All the events from old plot will eventually be in new plot, but at the moment there are some events that are not in new plot yet. So you can start any of them and then play the other one you haven't played