Continuing from where "The Poison Sky" left off, the TARDIS kidnaps the Doctor (David Tennant) and his companions Donna Noble (Catherine Tate) and Martha Jones (Freema Agyeman) and violently whisks them away to the planet Messaline in the midst of a generations-long war between humans and the Hath, fish-like humanoids. Upon leaving the TARDIS, they are confronted by armed men working for General Cobb (Nigel Terry), who force the Doctor's hand in a progenation machine, which uses his DNA to create an adult soldier within moments — Jenny (Georgia Moffett), the episode's titular character. Martha is subsequently captured by the Hath, whereas the Doctor, Donna, and Jenny are imprisoned by the humans because of the Doctor's pacifist attitude. Each of the primary characters learns about the war from its belligerents; the Hath and humans were initially meant to live on a peaceful colony, but were divided over a dispute about "the Source", believed by each side to be the breath of their creator. When the Doctor unwittingly reveals the location of the Source, the two sides race to claim it first.
The Doctor is initially dismissive of Jenny, his biological daughter, but becomes enamoured as the episode progresses. Donna is also distracted from the war by a series of numbered plaques on their journey. When they reach the location of the Source, it turns out to be a terraforming device within a colonising spaceship. Donna and the Doctor discover that the plaques represent the date building was completed, which was a mere seven days previous; the humans and Hath have bred so many generations through the progenation machines that their own history degraded into myth. The original casus belli was a power vacuum caused by the death of the mission commander.
Both the human and Hath forces converge at the Source concurrently. The Doctor declares the war to be over, and releases the terraforming agent; everyone present lays down their weapons, with the exception of Cobb, who tries to shoot the Doctor but Jenny steps in the way. After she collapses in the Doctor's arms, he finally tells her she is his daughter and that they have only got started. He tells her that they can go anywhere, if she holds on. She dies in his arms. Enraged, the Doctor holds Cobb at gunpoint, but refuses to shoot, asking the colonists to create a pacifist society.
At the end of the episode, the Doctor takes Martha home. Martha warns Donna that life with the Doctor can be dangerous, but Donna nevertheless resolves to stay with the Doctor indefinitely. Concurrently, on Messaline, Jenny revives in front of Cline and a Hath. She escapes Messaline, determined to follow in her father's footsteps by resolving disputes and fighting villains (and a whole lot of running).
Doctor David Tennant (Tenth Doctor)
Companions Catherine Tate (Donna Noble)
Freema Agyeman (Martha Jones)
Georgia Moffett – Jenny
Nigel Terry – Cobb
Joe Dempsie – Cline
Paul Kasey – Hath Peck
Ruari Mears – Hath Gable
Akin Gazi – Carter
Olalekan Lawal Jr. – Soldier
Writer Stephen Greenhorn
Director Alice Troughton
Script editor Lindsey Alford
Producer Phil Collinson
Executive producer(s) Russell T Davies
Production code 4.6
Series Series 4
Length 45 minutes
Originally broadcast 10 May 2008