This contains SPOILERS for "Frontier in Space", "Planet of the Daleks" and "The Green Death". Starting as a couple of missing scenes from "Frontier," the story goes AU at the end of "Frontier" and is basically a rewrite of "Planet", but should be understandable without having seen any of the episodes, although that might help.
I'm also using certain elements of New Who canon, such as the Time War, the Doctor's dislike for pears and a certain plot device from "The Sound of Drums".