Synopsis: The Doctor is depressed. Nine hundred years and what does he have to show for his life? No family, no stability, no roots. As he sits on a beach and ponders where he went wrong, a small mammal cries out to him telepathically.
The animal is a host for a virus-plagued culture. When the Doctor cures the host of the virus, he takes the animal to the ocean. There, he hopes the culture will be able to achieve its full potential and break the symbiotic ties to its host. Entering a brave new world of independence, the tiny cell culture profusely thanks the Doctor as its "savior".
His faith in the universe renewed, the Doctor returns to the TARDIS a changed, and more enthusiastic, Time Lord.