Review of Series 4, Episode 2: "The Fires of Pompeii"

First off, I'm still digging Donna.  It's great that The Doctor now has a companion who can challenge him.  She's more of an equal than his other recent companions and has no problem telling him when she thinks he's wrong.  Her questioning of him actually drove the plot of this episode, given that The Doctor was ready to cut and run early on when he realized where/when they were.  If Donna hadn't insisted on trying to help the people of Pompeii, the episode would have been a short one!  And how fun was the homage to Mary Poppins?!  (If you didn't catch it, when the mountain first trembles, Caecilius and his family run through around their house catching busts and vases just like the Banks family does in Mary Poppins when their neighbor, Admiral Boom, shoots his canon.)  It was also nice to see the sets from the HBO/BBC TV series Rome again; I kept watching for Titus Pullo in the background.

The plot seemed a little scattered and confusing, but I just kind of let it wash over me and enjoyed it for what it was.  Upon closer examination, I'm not sure the plot really holds together.  For example, the prophets can't see that Vesuvius will erupt in one day because the volcano is not "meant" to erupt thanks to the Pyroviles, who are draining the energy and keeping it from blowing.  (That is, The Doctor and Donna ultimately make Vesuvius erupt to prevent the Pyroviles from taking over the Earth, thereby changing what was "supposed" to happen.)  That's all fine and dandy and plays into the idea that The Doctor can't always change history for the better; in fact, he sometimes has to change history for the worse to prevent a greater disaster from occurring down the line.  However, if Vesuvius was not "meant" to erupt, then how did the prophets gain the ability to see the future in the first place (since this ability was explained as a side effect of a rift in time that resulted from Vesuvius uber-powerful explosion)?  If Vesuvius didn't erupt in the original timeline, there would have been no explosion to create the rift in time that gave the prophets their precognitive powers.  Whew!  My head hurts....

There was another reference to Rose returning (when Lucius tells The Doctor that "she is returning").  Is she this season's Bad Wolf?  Lucius also mentions that Donna has "something on her back."  I'm not sure what that meant.  Any ideas?  Another continuing theme this season seems to be missing/destroyed planets.  Last episode, there was mention that the Adiposian home world had been taken (which is why the Ms. Foster was seeding the children on Earth).  In this episode, the Pyroviles' planet has been taken as well.  So, who or what is taking these planets?  Sontarans?