Corner of the Eye (short story)

Summary
Bob and Kathy are emailing each other and discover that they are not who they say they are as they find the doctor tells Bob that Kathy died in the celler and is a floof an alien who has taken her identity and the same thing has happened to bob.

Characters

 * The Doctor
 * "Bob"
 * "Kathy"
 * Floof

Continuity
As Rose does not join the Doctor on this adventure it could take place either around the time of Attack of the Graske or somewhere between Doomsday and Smith and Jones.