Isomorphic? I don't think so

Dear Johnny Fanboy,

In the Tom Baker Doctor Who serial Pyramids of Mars, the Doctor avoids being killed by Sutekh's minions by claiming that the controls of his TARDIS are isomorphic, meaning they respond to him alone. The Doctor cannot be lying about this, because Sutekh has already proven that he can read the Doctor's mind.

So how come we've seen people other than the Doctor operating the TARDIS both before and after this story?

Geoff Cunningham

Johnny Fanboy replies:

This isn't the only safety feature of the TARDIS to come and go over the years. Just think of the HADS (Hostile Action Displacement System), which the Doctor activates in The Krotons but then never again throughout the course of his television adventures. Then there's the TARDIS interior's state of temporal grace, which, in The Hand of Fear, supposedly prevents weapons from being fired, but which ceases to function at some point prior to The Invasion of Time. As the Doctor admits, when Nyssa questions him about it in Arc of Infinity, "Nobody's perfect." Even the ship's force field fails some time before Bad Wolf/The Parting of the Ways, forcing the Doctor and Captain Jack to rely on an alien device, Margaret the Slitheen's tribophysical waveform extrapolator, instead.

We can similarly assume that the isomorphic restraints are deactivated, either accidentally or deliberately, at some point after Pyramids of Mars - certainly before The Face of Evil, at the end of which Leela causes the TARDIS to dematerialise. Perhaps this occurs when the Doctor switches control from the white-walled number two control room to the older, wood-panelled control room. Alternatively, maybe the Time Lord decides that the isomorphic restraints are useless (after all, they didn't do much good against Sutekh) and turns them off for that reason.

But why switch them on in the first place? I theorise that the Doctor decides to "child lock" the controls after Harry "clumsy, ham-fisted idiot" Sullivan sends the TARDIS off course when he fiddles with the helmic regulator at the start of The Ark in Space.

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