The Doctor and Martha go to the frozen planet of Agelaos after the Doctor discovers a message warning people to stay away from the planet. Being curious, the Doctor lands the TARDIS on the planet. There, as they are walking around the planet, Martha and the Doctor are chased by strange mutants. They are rescued by a man named Waechter who is connected to the lighthouse beacon that is keeping the warning message running.
Waechter explains to them that humans colonised Agelaos for the power from the wormhole which was above the planet. Waetcher also explains that he is the last human alive on the planet, that he has been keeping the beacon in good repair for 500 years, and that this is possible because of a chip in his neck which connects him to the beacon. This chip keeps his metabolism in check so that he will not die.
The Doctor, obliging to Waetcher's wishes, finds a way to disconnect him from the beacon so he can go home to Earth. As soon as the chip is disconnected, Waetcher starts to die. They bring him to the TARDIS where Waetcher begins to mutation into one of the strange creatures. The Doctor reveals that the humans on the planet did not die, they were merely mutated by the millions of types of alien DNA filtering through the wormhole. Waechter's chip was the only thing keeping him human, so the Doctor gives him the choice of reconnecting the chip, meaning he would have to stay on the planet, or completing the mutation. Waechter chooses the latter and they return him, fully mutated, to Agelaos and the rest of the mutated humans.
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- An audio version of this story was released separately from The Story of Martha purchasable by download only, on 11 June 2009.
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