"The Doctor's Daughter" is the sixth episode of the fourth series of the revival of Doctor Who. It stars David Tennant as The Tenth Doctor, Catherine Tate as Donna Noble, Freema Agyeman as Martha Jones, and Georgia Moffet as Jenny, the Doctor's titular daughter.
This episode starts off right where the last one lets off: The Doctor, Donna and Martha are in The TARDIS, which starts to shake and take off with no input, bringing them to an alien planet where the Doctor's tissue is sampled, and a fully formed daughter steps out of a machine three seconds later. And that is before the opening credits.
We soon find out that the planet we are on is stuck in the middle of a generations long war between two societies, one of humans and the other of fishlike aliens. The Doctor finds their endless war repugnant, but his daughter Jenny is a natural warrior, raised by the machines that produced her to fight and kill without question. The war becomes more violent and both races might be wiped out if The Doctor can't solve the mystery in time. Along with having to stop the fighting, The Doctor has to cope with his daughter's worldview, as well as challenges to his own self-perception as a pacifist.
The story ends on several different notes, with a plot twist (perhaps) taken from the short story "Frost and Fire" by Ray Bradbury, a revealing moment about The Doctor's psychology, and an even further plot twist that, five years later, has yet to be resolved.
This is a great episode, and is especially great when seen in the light of the rest of Series 4, which has a number of episodes dealing with The Doctor's competing roles as healer and warrior. It also hearkens back to Series One's The Empty Child, where denial of an offspring and what they represent is the hinge of the plot and theme. It also, for good measure, features Georgia Moffet, the daughter of 5th Doctor Peter Davison, playing the daughter of David Tennant, the man she would later marry. (You may or may not want to think about that one.)
In short, another thought provoking and well plotted episode.