2. Human Nature/The Family Of Blood (Tenth Doctor)
David Tennant's single greatest performance, and the second episode written by the genius Paul Cornell, who had previously given us Father's Day (#25 on this list). The story begins in England in 1913, where a man named John Smith awakens from a strange dream, wherein he is in fact an alien with two hearts, flying around the universe in a blue box that is somehow larger on the inside. John Smith - a teacher in a boys boarding school - begins writing down and drawing all his dreams in something he calls "Journal Of Impossible Things". He shares these with a nurse named Joan Redfern, with whom he begins quickly falling in love. Witnessing all this is Martha Jones, the maid for John Smith. Obviously something is up. The audience soon learns that the Doctor was attempting to escape from The Family of Blood, a family of creatures that want to feed off the Doctor, and in so doing gain everlasting life. They can trace the scent of a Time Lord across a galaxy, so to escape them the Doctor converts himself into a human, in so doing removing one of his hearts and deleting his memories. And thus, he becomes John Smith. The final impact of the story comes when he must choose between being John Smith, and possibly losing everything he loves, or being the Doctor, and definitely losing everything he loves.