I've not been watching Dr. Who long, really just during the David Tennant and Matt Smith tenures as the Doctor. My understanding has been that a major plot point is that Dr. Who isn't supposed to cross his own time line - to meet himself at the same point in time. However, in the recent series finale, he does that several times. Do I have it wrong? Or did the finale plot allow this somehow? If he can cross his own time line, why doesn't he go back and save people who have died along the way?