Doctor Who: Season 4, Episode 6

4 - 6: The Doctor's Daughter

The Doctor, Donna, and Martha are drawn to a world at war. Two factions, human beings and the monstrous Hath, seek the mysterious "source" at the heart of their underground city. Soldiers on both sides are genetically bred for battles using genetic extrapolators, and when the Doctor is subjected to one of these devices, it produces a unique soldier... and a very special girl...