The Cartmel Masterplan is a fan name for the planned Doctor Who backstory developed primarily by Andrew Cartmel, Ben Aaronovitch, and Marc Platt, which they intended to restore some of the mystery of the Doctor's background that had been lost through revelation of the existing backstory. Although hints were dropped in the last two seasons, the proposed revelations never materialised on screen as the programme was taken off the airwaves in 1989.
Some of the stories during the Seventh Doctor's tenure were intended to deal with the lack of mystery by suggesting that much of what was believed about the Doctor was wrong and that he was a far more powerful and mysterious figure than previously thought. In an untelevised scene in Remembrance of the Daleks, the Doctor stated that he was "far more than just another Time Lord." In Silver Nemesis, lines about the creation of validium and Lady Peinforte knowing the Doctor's secrets were meant to point towards this mystery.
The suspension of the series in 1989 meant that none of these hints were ever resolved. The "Masterplan" was used as a guide for the Virgin New Adventures series of novels featuring the Seventh Doctor, and the revelations about the Doctor's origins were written into the novel Lungbarrow by Marc Platt.
The Other 
Cartmel felt that years of explanations about the Doctor's origins and the Time Lords had removed much of the mystery and strength of the character of the Doctor, and decided to make the Doctor "once again more than a mere chump of a Time Lord". To do this, he built on the established mythology of the Time Lords:
...Omega and Rassilon were the founding fathers of Gallifrey. They towered above the Time Lords who followed. They were demigods. [Dialogue in Silver Nemesis was] a subtle attempt to say that there was a third presence there in the shadowy days of Gallifrey's creation. In other words, the Doctor was also there. So he's more than a Time Lord. He's one of these half-glimpsed demigods.
More of these hints would have been seen in the story Lungbarrow, which was at one point planned for the programme's 1988–89 series. Lungbarrow would have been set in the Doctor's ancestral home on Gallifrey, but the story was abandoned; elements were adapted into the broadcast story Ghost Light, but none of the Time Lord backstory survived the transformation. However, author Marc Platt eventually adapted his ideas for Lungbarrow (along with other elements of the "Masterplan") into a Doctor Who novel for the Virgin New Adventures series.
The Other was first mentioned explicitly in the novelisation of Remembrance of the Daleks by Ben Aaronovitch as a shadowy figure in Time Lord history, one of the founding Triumvirate of Time Lord society after the overthrow of the cult of the Pythia that had, until then, dominated Gallifrey. The other two members of the Triumvirate were Rassilon and Omega.
Of the three, the Other's origins are the most obscure, with the circumstances of his birth and appearance being a mystery. Like Rassilon, various contradictory legends surround the Other, some hinting that he had powers surpassing that of Rassilon or Omega, and some even suggesting that he was not born on the Time Lords' home world of Gallifrey. Even his name is lost to time, which is why he is simply referred to as "the Other". A minor Gallifreyan festival known as Otherstide is celebrated yearly in his honour.
Susan Foreman 
Retconning the Doctor's backstory was complicated by the existence of Susan Foreman, who was presented in the television series' earliest days as the Doctor's granddaughter. In different versions of the "Masterplan", different explanations were presented to deal with this: according to writer Lance Parkin, in the "original" version of the Masterplan, the Doctor rescued Susan as an infant from Gallifrey's recent past.
According to the novel of Lungbarrow, Susan Foreman was the granddaughter of "the Other," who may have been reincarnated as the Doctor. The Doctor, in his first incarnation, travelled back to the dawn of Time Lord civilization and rescued a teenaged Susan, who recognised him as her grandfather. The Doctor did not initially recognise her, but knew that this was somehow true. This version of Susan's origins is reflected in many Doctor Who spin-offs.
- Cartmel, Andrew (2005). Script Doctor: The Inside Story of Doctor Who 1986–89. London: Reynolds & Hearn. pp. 134–135. ISBN 1-903111-89-7.
- See for instance, The Handbook, Howe, Stammers & Walker, (Telos 2005). pg. 726.
- Parkin, Lance; with additional material by Lars Pearson (2007). AHistory: An Unauthorized History of the Doctor Who universe (2nd ed.). Des Moines, Iowa: Mad Norwegian Press. p. 380. ISBN 978-0-9759446-6-0.
- Cartmel, Andrew (2005). Script Doctor: The Inside Story of Doctor Who 1986–89. London: Reynolds & Hearn. pp. 171–172. ISBN 1-903111-89-7.
- Parkin, Lance; with additional material by Lars Pearson (2007). AHistory: An Unauthorized History of the Doctor Who universe (2nd ed.). Des Moines, Iowa: Mad Norwegian Press. pp. 396–398. ISBN 978-0-9759446-6-0.