S04E06: 'The Doctor's Daughter'

The Doctor, Donna and Martha arrive on the planet Messaline, and the Time Lord's DNA is quickly taken via his hand by a group of war-hungry humans. Instantly, via progenation, his 'daughter' is created via a machine and is armed with a gun. The humans are fighting against the humanoid fish-like Hath, who communicate via bubbles. The Doctor's daughter, soon dubbed Jenny, blows up an explosive device that isolates Martha in the Hath quarters.

Martha strikes up a friendship with one of the Hath after helping relocate its shoulder. The pair soon embark on a random trek on the outer surface of the planet, where Martha falls into some quicksand. The Hath jumps in and pushes her out to safety, but loses its own life.

The Doctor and Donna come into contact with General Cobb, who is determined to destroy the Hath and says that many generations have fought in the war. Both Cobb and the Hath are after 'The Source' - something that gives 'the breath of life'. The Doctor finds a secret lair on the computerised map that shows the way to The Source, with both humans and Hath seeing the new tunnels via their monitor.

The time travellers desperately try to talk Jenny into adopting a pacifistic attitude and soon the trio is imprisoned. Jenny snogs one of the guards from inside her cell and facilitates the trio's escape. They count down a numerical sequence on the walls in the hidden tunnels on their way to the source, helped by Jenny fending off Cobb's troops. Donna later notes that the numbers are dates, and that the war has only been going on for a matter of days.

The Doctor discovers that The Source is located in a spaceship and Martha joins the trio inside. They find a mini-forest with a terraforming device. The humans and Hath both break in at the same time and The Doctor tells them the truth of their existence, demands peace and shatters the terraform device to release the gasses and create a new world.

Cobb raises his gun and Jenny jumps in the way, only to die in her father's arms. The Doctor puts a gun to Cobb's head, but says he would never pull the trigger. The time travellers depart and Martha is returned home. Meanwhile, Jenny comes back to life and flies off in a shuttle.