6. Partners in Crime
The opening to Series 4 is strange, to say the least. Bemusing, if you will. People turning into fat? It seemed very amusing when I was 12, but looking back, it was one of the weaker opening episodes. However, the reason for this episode being high up on the list is the reintroduction of Catherine Tate to the show. The plot, which revolves around Donna Noble investigating Adipose Industries, who are selling a miracle pill which reduces people's weight - but all is not as it seems. This provides a mere backdrop to one of the funniest episodes of the revived series, and begins the chemistry which will exist between Time Lord and Companion for the rest of the series. The best example of this is definitely when the Doctor and Donna finally meet midway through the episode, during opposing investigations. Donna spies the Time Lord through a window, and the Doctor's reaction is truly priceless. It is clear that whilst retaining some of the humor of her original character in The Runaway Bride, Tate has made Donna a more rounded, human person - coupling the humor with regret, which is a nice parallel to the Doctor himself. The Doctor refrains from killing the Adipose, showing that he has become a more human character as well. All this combines to make an entertaining, if weakly-plotted, episode.