I'll give a rundown of what I know.
The Eleventh Doctor, and former companion
Jo Grant will appear in an episode written by Russell T. Davies.
They filmed Season 4 and half of 5 together.
The episode titles are:
The Nightmare Man - Written by Joseph Lidster
The Vault of Secrets - Written by Phil Ford
Death of the Doctor - Written by Russell T. Davies (The episode(s) 11 and Jo appear in)
The Empty Planet - Written by Gareth Roberts
Lost in Time - Written by Rupert Laight
Goodbye, Sarah Jane Smith - Written by Gareth Roberts and Clayton Hickman
Luke Smith and K9 leave the show after "The Nightmare Man"
Here is the cover of the novelization of "Death of the Doctor"
