Here's a story Dr. Who fans have waited decades to see. The Doctor warned us back us back in the early 80s that when time travelers tried to change their own personal histories, Very Bad Things happened. Bad Things that even a renegade Time Lord didn't want to mess with. So what kind of Bad Things were they?
Now we know.
Eccleston's tight-lipped "say nothing more than is necessary" Doctor is the real culprit here. Bad enough he can't even bring himself to tell her the name of his people's planet, here his failure to go over the basic safety rules of meeting yourself in the past and his willingness to bring three Roses to the same time set up an event that threatens all existence.
Rose's father died when she was a baby. She wants to go back in time to see him, and thanks to the Doctor's failure to explain the rules about this sort of thing, ends up saving his life in such a way that it causes a rift in time. This rift allows the Very Bad Things to come through, and they are Very Bad indeed. The Doctor tries some fancy moves to save everybody and fails. Rose's Dad tries a simple move, and sacrifices himself to save everyone else.
When the Doctor explained to Rose that "his people" used to prevent this sort of thing but now they're gone, I felt my first touch of fear. So who's protecting the time stream now? And what will happen if nobody is?
It's a simple, beautiful episode, with incredible acting. I was crying by the end of it. The current season seems to have a theme running through it that everyone has the potential within themselves to be a hero, if they would just reach inside past all the banalities of their day-to-day world and find their true strength. The Doctor happens to be very good at enabling other people to realize this truth. In the course of trying to get this message across the story has the Doctor getting saved by other people a lot, but I can put up with a lot to hear one of my favorite messages getting out. Besides, it makes the stories more interesting when you get to guess exactly which person (or what combination of people) is going to save the world this time. Ensemble shows are so much livelier that way, and for the first time since the 60s Dr. Who is starting to look like an ensemble show.