Countdown to 60: "They sleep in my mind"

No. 14.

As the opening episode of Season 5, producer Innes Lloyd and his script editor / heir apparent Peter Bryant elected to treat Tomb of the Cybermen as a soft relaunch for the series.

This was the beginning of Patrick Troughton's first full season, accompanied by a brand new companion - Victoria - and the series was no longer going to be partially defined by the Daleks.

This is why we get the lengthy TARDIS scene, unnecessary to the main plot, in which the Doctor tells Victoria - and new viewers - of the basic premise of the series. 

This scene takes place in a wonderful TARDIS set - the best it has looked since The Edge of Destruction. This is thanks to it being filmed at Ealing, because director Morris Barry didn't want to take up valuable studio space with the ship just for one scene when recording this opening episode.

The script goes a step further by having the Doctor address the subject of his age for the first time. This has proven to be a contentious continuity issue - with the Third suggesting he's thousands of years old, the Fourth settling for the mid-700's, and the Seventh being specifically 953 on his debut - despite the Tenth claiming to be only 900.

As always, fans have thought about this long and hard and there is a way round it. The Doctor here claims to be "about 450" in Earth terms, and that's after having to think about it for a moment or two.

I think he's having to convert it from Gallifreyan years to Earth ones, for the benefit of his two pre-20th Century human companions. There's absolutely no reason why Gallifrey has to have the same period of rotation / solar orbit duration as the Earth.

This difference then helps explain how he can be 953 one day, and only 900 much, much later. The 953 comes from Time and the Rani, where he says that he shares the same age as the Rani - i.e. another Time Lord. They are both using Gallifreyan years. The 900 comes from Voyage of the Damned, where he is talking to inhabitants of Sto - so presumably converting his age into terms they will understand. 

Maybe.

Not content with establishing the Doctor's age (roughly), the script for Tomb then goes on to discuss his family. This is still part of the move to establish Victoria as the new companion, as she has just lost her father to the Daleks and so become an orphan, but also to tell new viewers more about the Doctor.

When we first met him, he was travelling with his granddaughter Susan. That she is his blood relative is never questioned until long after she has left the series. At the time, there was no mention of her parents - one of whom must be a child of the Doctor if Gallifreyan reproductive biology mirrors that of human beings. (We'll later hear that it does). He and Susan talk about the possibility of going home at some point - but only ever mention the place, not the people who might be waiting for them there.

The Doctor tells Victoria only that he has family connections. It isn't clear if they are still alive or if they are dead from the wording he uses. 

The fact that he and Susan never mentioned anyone back home suggests the latter.

The Doctor says that he has to consciously think about his undefined family to bring them to the forefront of his mind - otherwise they "sleep" in his memory. Again, the implication is that they go back a long way, and they may be long dead. He doesn't mention Susan at all, so it isn't clear if he's counting her as part of this group or not.

As mentioned, the problem of Susan as a fully paid up member of the Doctor's own race, and a blood relative, arises after she has departed from the narrative - and the first time it does so is when we consider his age, as here. 

David Campbell is an ordinary human being of terrestrial origins, so has a life span of the usual three score years and ten, or thereabouts. What is going to happen when his wife gets to 70 and still looks like a teenager? What would his reaction be if she fell off a cliff and turned into another woman - or even a man? What would the children be like, assuming they had some?

Obviously, at the time no-one would have thought of this, but it worries fans later on. We also have the apparent apathy of the Fifth Doctor when he's reunited with her in the Death Zone. Not the reaction of someone seeing their only living relative for the first time in centuries.

In The Pilot, however, the Doctor has Susan's photograph in pride of place on his desk - the only other picture being of his "wife" River Song. The new series certainly seems to think that she was indeed his granddaughter.

The only other reference to family in recent years is when the Doctor told Rose Tyler that he was a father once, which we must assume refers to Susan's father or mother. 

The mysterious woman seen in The End of Time is assumed to be his mother, still alive on Gallifrey up to the last day of the Time War at least, but this has never been officially confirmed on screen.

Since the Timeless Child nonsense, however, the universe could be teeming with his children, grandchildren, great-grandchildren...