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
Directed by Alice Troughton
Written by Stephen Greenhorn
Script editor Lindsey Alford
Produced by Phil Collinson
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 4.6
Series Series 4
Length 45 minutes
Originally broadcast 10 May 2008
Continuing where "The Poison Sky" left off, the TARDIS takes the Doctor, Martha, and Donna to the planet Messaline. As they emerge from the TARDIS, they are met by soldiers working for General Cobb. The soldiers force the Doctor to stick his hand into a progenation machine, which uses his DNA to generate a female soldier who becomes the Doctor's daughter. They are soon confronted by the other occupants of the planet, the Hath. The Hath attack, taking Martha hostage. The Doctor's daughter causes an explosion that seals off the corridor they are in, trapping Martha on the other side. The Doctor and Donna are taken to see General Cobb in the command center, and along the way Donna names the Doctor's daughter "Jenny" because the Doctor calls her a generated anomaly.
The Hath as they appear at the Doctor Who Experience.
On the other side of the corridor, Martha tends to an injured Hath and earns their trust. The Hath take her back to their command center. Meanwhile, the Doctor and Donna meet with General Cobb. The General explains that, initially, they were meant to live with the Hath, but a dispute arose over something called "the Source". Both sides believe that this Source is the breath of their Creator, and the Doctor inadvertently reveals its location to Cobb. On the Hath side, Martha also unknowingly interprets the location of the Source and the two sides prepare for battle. Cobb tries to convince them to join the fighting, but imprisons them when they refuse. Jenny uses her feminine charms to incapacitate the guard and they escape their cell.
The Doctor initially dismisses Jenny as an echo of what it means to be a Time Lord, but as they spend more time together he begins to accept her. The Doctor confides privately to Donna that he is resistant to Jenny coming with them in the TARDIS because she reminds him too much of Gallifrey and everything he lost in the Time War. Donna also becomes intrigued by a series of numbered plaques that she notices in each room. When they reach the location of the Source, it turns out to be a terraforming device within a colonising spaceship. Donna realises that the plaques represent the dates each part of the 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 has degraded into myth. The original cause of the conflict is determined by the Doctor to have been the death of the mission commander.
Meanwhile, Martha has been making her own way with one of the Hath to the Source. They are traveling on the surface of the planet when the Hath chooses to sacrifice itself to save Martha from quicksand. Devastated, she reunites with the Doctor and Donna near the Source shortly before both armies arrive. The Doctor declares the war to be over and releases the terraforming agent. Everyone present lays down their weapons except for Cobb, who tries to shoot the Doctor. Jenny steps in the way and takes a bullet to the chest, and the Doctor holds her as she dies. After mourning the loss of his daughter, an enraged Doctor picks up Cobb's gun and holds it to his head, but refuses to shoot him. He angrily throws the gun aside and declares that the humans and Hath should build their new society based on peace.
Jenny's body is taken to be given a proper burial, and the Doctor and Donna take Martha home. Martha says she can't handle the death and devastation any more and warns Donna that life with the Doctor can be dangerous. Donna nevertheless resolves to stay with the Doctor indefinitely. As Donna leaves them to walk on their own, the Doctor and Martha reminisce and Martha expresses her sympathy to the Doctor about losing Jenny. He and Martha share a hug before she runs inside her home. Meanwhile, on Messaline, Jenny suddenly revives. She then commandeers a rocket and leaves the planet.