Doctor Who: The Waters of Mars

Doctor Who: The Waters of Mars is a special episode of the science fiction television series aired during a transitional period when Doctor Who changed show runners from Russel T. Davies to Steven Moffat. The episode was written by Russell T Davies and Phil Ford and directed by Graeme Harper. The third special in the short season, The Waters of Mars featured the title Doctor, played by David Tennant, arriving in the future at a human colony on Mars.

An alien hero of the universe, the Doctor realizes that though the colony is in peril, there is nothing he can do to save it. The colony is meant to be destroyed, but its destruction leads to greater discoveries for the human race. If the Doctor prevents the colonists' deaths, it will mean stunting the future of humanity. In spite of his initial apprehension, the Doctor is drawn into the mystery of a virus carried in water infecting the colony's small crew. They discover that the sentient virus intends to get to earth, which the colony's captain refuses to allow. After telling the captain what is supposed to happen to her crew, the Doctor abandons the colony.

Although the colonists try to escape on a rocket, they find out that more crew members have become infected and must destroy their means of escape. With time running out for the remaining trio, the Doctor reappears to rescue them from their infected crew mates. The captain programs the colony to self destruct, and the Doctor returns the survivors to Earth. Rather than being appreciative, the Captain questions the Doctor's motives in saving them knowing it would alter an entire species' future. He implies that, with his own species decimated, he no longer needs to follow their rules. The captain disagrees and kills herself, setting the human race back on its predestined path. The Doctor realizes that with his loss of boundaries, he is nearing his own inevitable death.