Synopsis: This story unusually takes place entirely in the TARDIS. As the Doctor and his companions travel deeper into the TARDIS, they go further into the ship than has ever been visually depicted before or since, in any medium. Deep at the heart of the TARDIS, they even find the TARDIS memory banks, or what are essentially the Doctor Who equivalent of Star Trek's holodeck. Significantly, this occurred prior to the proper introduction of the Holodeck in Star Trek: The Next Generation.
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.