The Doomsday Contract (in-universe)

The Doomsday Contract was an unproduced Doctor Who serial written by John Lloyd. It was later released as an audio play by Big Finish, starring Tom Baker, Lalla Ward, John Leeson, and Julian Wadham, and was written by John Lloyd and Nev Fountain. (WC: Tom Baker stars in John Lloyd's lost Doctor Who adventure, The Doomsday Contract)

Important note
This is a in-universe counterpart of the real world Big Finish audio drama The Doomsday Contract that exists within the Doctor's universe. It was the central concept that the webcast Tom Baker stars in John Lloyd's lost Doctor Who adventure, The Doomsday Contract was based around. The webcast, while telling the real world story of the audio drama, also presented an abundance of evidence that the events transpired in-universe as well, as among other things, the narrator himself was reading from the Doctor's Five Hundred Year Diary.