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