The TARDIS takes the Tenth Doctor, Martha and Donna to the planet Messaline. As they emerge from the TARDIS, they are met by human soldiers who force the Doctor to stick his hand into a progenation machine. The Doctor’s daughter, whom Donna later names Jenny, emerges from the machine. They are soon confronted by the other occupants of the planet, the Hath. The Hath take Martha hostage. Jenny causes an explosion that traps Martha on the other side of the corridor. Martha tends to an injured Hath and earns their trust. The Doctor and Donna meet with General Cobb in the command centre. Cobb explains that, initially, the humans were meant to live with the Hath, but a dispute arose over something called “the Source”, which Cobb believes has the power to end the war by wiping out the Hath. The Doctor inadvertently reveals the location of the Source to the humans and the Hath. The two sides prepare for battle.
The Hath as they appear at the Doctor Who Experience.
The Doctor confides privately to Donna that he is resistant to Jenny travelling with them because she reminds him too much of everyone who was killed in the Time War. Donna becomes intrigued by a series of numbered plaques that she notices in each room. The Doctor, Donna, and Jenny reach the Source, which is a terraforming device within a colonising spaceship. Donna realises that the plaques represent the dates each part of the building was completed; according to the dates, the ship landed only seven days ago. The humans and Hath have bred so many generations through the progenation machines that their own history has degraded into myth. The Doctor determines the original cause of the conflict was a power vacuum created after the death of the mission commander.
By travelling over the surface with a Hath’s help, Martha 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. Cobb 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. The Doctor picks up Cobb’s gun and holds it to his head, but refuses to shoot him. He declares that the humans and Hath should build their new society based on peace. Cobb is demoted, arrested, and imprisoned for Jenny’s murder, and Martha returns home. Meanwhile, on Messaline, Jenny suddenly revives. She then commandeers a rocket and leaves the planet.