In each series, there is an underlying story arc that pans all episodes until finally fully addressed in the season finale. For Series 1, it is the concept and identity of the Bad Wolf. Series 2, the Torchwood Institute. Series 3, the identity of Mr. Saxon. In series 4, there are several little references and jokes that eventually lead to the plot of the finale, including the missing planets and the bees disappearing from Earth, and repeated appearances of Rose Tyler on screens and monitors. In addition, there are repeated references to there being something on Donna's back. While this does not carry on to the series finale itself, it does play an important part in the episode which sets up the finale's story. In series 5, there are "cracks" in the universe, which must ultimately be fixed in the series finale.
The Tenth Doctor was the tenth incarnation of the renegade Time Lord known as the Doctor. Unlike his immediate predecessor, who was plagued by melancholy due to his actions during the Last Great Time War, this incarnation was much more outgoing and genial, a demeanour that hid the "survivor's guilt" that had plagued him since the war. Over his life, however, the weight became much more pronounced. He was haunted by companions lost, enemies believed long-dead and events that threatened the fabric of time itself. He met his end after he absorbed a huge quantity of nuclear radiation whilst saving the life of his friend, Wilfred Mott, leading to his regeneration into his next incarnation.