The TARDIS lands on the planet Messaline. The Doctor (David Tennant), Donna (Catherine Tate), and Martha (Freema Agyeman) emerge and are met by human soldiers who force the Doctor to stick his hand in a progeneration machine. The Doctors cloned daughter (Georgia Moffett) emerges from the machine.
Then they meet the fish-like Hath, who take Martha hostage. The Doctor’s daughter, who Donna names Jenny, causes an explosion that traps Martha. She tends to an injured Hath and earns their trust. The Doctor and Donna meet with General Cobb (Nigel Terry) in the Command Centre. He tells them that the humans were originally supposed to live with the Hath, but they argued over the Source, which could wipe out the Hath. The Doctor accidentally reveals the location of the Source and the two factions prepare for battle.
The Doctor tells Donna that he is reluctant to travel with Jenny because she reminds him of the victims of the Time War. Donna notices the number plaques in each room. The Doctor, Donna, and Jenny find the Source, which is a terraformation machine in a colonising spacecraft. Donna realises that the numbered plaques represent the completion dates of each room, which means the ship landed only seven days ago. The humans and the Hath bred so many generations through propagation machines than their own history has become myth. It all happened with the death of the mission commander.
With help from the Hath, Martha travels over the surface and reunites with the Doctor and Donna near the Source, arriving before the competing armies. The Doctor declares the war over and releases the terraformation agent. Cobb tries to shoot the Doctor, but Jenny steps in front of him, taking the bullet, and dies in the Doctor’s arms. The Doctor grabs the gun but is unable to shoot Cobb. He orders the humans and the Hath to build their civilisation on peace. Cobb is imprisoned for Jenny’s murder and Martha returns home. Chip off the old block, Jenny comes back to life, steals a spaceship, and leaves the planet.
The story was filmed at Plantasia Botanical garden in Swansea. It received mixed reviews, reminding some reviewers of the Tom Baker years, with people running down corridors and two warring factions for the Doctor to unite. Actors playing the fish-like Hath, though they spoke only in gurgles, had written dialogue to study so they could move properly. The two Hath principles were Hath Peck and Hath Gable. Georgia Moffett, who plays Jenny, had previously tried out for the role of Rose Tyler. She is the daughter of Fifth Doctor Peter Davison and actress Sandra Dickinson, who appeared in Hitchhiker’s Guide to the Galaxy. A year after this episode aired, Moffett and David Tennant started dating and eventually married, making David Tennant Peter Davison’s son in law, Moffett the wife and the daughter of the Doctor, and the child born later the daughter and granddaughter of the Doctor.