When the TARDIS appeared to explode into a million pieces under a barrage of Dalek energy beams on Saturday night, fans were probably about as worried as they were last week when Clara and Missy were apparently vaporised by the tinpot terrors.
We now know the blue box is indeed safe (just like the Doctor’s two best friends) – but how exactly did it survive? Four words: Hostile Action Displacement System.
The HADS, as the Doctor often likes to call it, is a TARDIS defence mechanism which – when he actually remembers to switch it on – sees the TARDIS automatically jump to a nearby safe location when it comes under threat (given that he did decide to engage it this time, the Doctor clearly thought a trip to meet Davros could potentially contain some mild peril…)
The HADS was first referred to in 1968 story The Krotons and was used a couple of times by the Fifth Doctor – once in a plan that foiled the Daleks, so they really should have seen it coming this time – and intermittently after that.
It was first referenced in New Who when the 11th Doctor sent the TARDIS to safety from a sinking submarine near the North Pole in series seven episode Cold War. The TARDIS reappeared a short while later… but at the South Pole. In seres eight’s Kill the Moon, when the 12th Doctor assured Clara the Tardis would “turn up somewhere” she reminded him “last time you said that it ended up on the wrong side of the planet.”
So the fact that it returned to the same place on Skaro this time suggests it’s working better than ever…