Last mentioned in the Wild Blue Yonder when the TARDIS needs to recover from having coffee spilt into its systems, the Doctor talks about the HADS — a defence mechanism that engages when the TARDIS is under attack and moves itself and or its occupants to somewhere safe. But what exactly are the HADS and when did the Doctor first mention them?

The HADS, which stands primarily for the Hostile Action Displacement System, are designed to move the TARDIS out of harm’s way until it’s safe to return. They have to be switched on and generally spend their time switched off; otherwise the TARDIS would just vanish every time the Doctor landed somewhere. However, there have been a few times on screen that the HADS have kicked in and left the Doctor and his companions stranded until they solve the threat.

The HADS are first seen in action in the Second Doctor adventure, The Krotons. When the Krotons surround the TARDIS, they subject it to a barrage of fire and much to Zoe’s shock, she sees the TARDIS vanish. Luckily though, the Doctor tells her that he has been fiddling with the systems and switched the HADS on: the TARDIS is perfectly safe — it’s just moved away from the danger and it quickly rematerialised when the Krotons moved away.

Surprisingly in the classic series, this was the only time the HADS were mentioned, though in the Target novel for Time and the Rani, it is stated the reason why the Rani is able to shoot down the TARDIS and then bring it to her planet in a tractor beam is because the Doctor hadn’t engaged the HADS.

It’s not until we get to the modern series that the HADS made a return in the 50th anniversary year.

(Before that, The Parting of the Ways introduces Emergency Programme One which can be activated if a travelling companion is in a very dangerous situation and can bring the TARDIS and the companion home. It’s a similar device to the HADS, though in theory the TARDIS can no longer return even when a location is eventually safe.)

In 2013’s Cold War, the TARDIS leaves the Eleventh Doctor and Clara stranded on a Russian submarine in 1983 in the middle of a missile crisis while an Ice Warrior is being resurrected on board. However, unlike The Krotons where the TARDIS appeared back where it had left, it instead reappeared at the South Pole instead of the North Pole where the Doctor and Clara were.

Clara also got to see the HADS in action during the Twelfth Doctor story The Magician’s Apprentice/The Witch’s Familiar when the TARDIS is seemingly destroyed by a barrage of Dalek gunfire. However, when fleeing Skaro, the Doctor uses his sonic sunglasses to reassemble the TARDIS to take him and Clara out of harm’s way — this was later called the Hostile Action Dispersal System.

The HADS were last mentioned in Wild Blue Yonder. When Donna spills her coffee into the TARDIS’ new central console and makes everything blow up, the damaged TARDIS brings them to a seemingly abandoned spaceship on the edge of known space in the far future. The Doctor has to engage a complete system reboot which activates the HADS too. Because of the presence of the Not-Things, the TARDIS leaves the Doctor and Donna stranded until they solve the problem. The spaceship is about to explode — only then does the TARDIS return because it knew the Not-Things couldn’t stop the self destruction in time and the hostile actions had ended.

But the HADS have also been used sparingly in the extended media too. The Suns of Caresh sees the Third Doctor lose the TARDIS when it is accidently triggered by a hailstorm. The Doctor is forced to explain to Jo Grant that he accidently set the settings for the HADS up too high.

The Fifth Doctor would use them twice, once in the Big Finish audio adventure, The Mutant Phase when he is forced to move the TARDIS so that he doesn’t meet himself, and later in the audio drama, Renaissance of the Daleks, in which he has to trick the HADS into working in order to defeat the Daleks.

Like the Fifth Doctor, the Seventh Doctor would also see the system coming into action twice — once in the Big Finish audio drama, The Angel of Scutari which saw the TARDIS hit by a cannon ball in 1854 which dispersed its outer plasmic shell and led into the huge Black and White TARDIS arc that culminated with Gods and Monsters. Ace would be instructed by the Doctor to activate the HADS to move the TARDIS out of danger in the Virgin New Adventures book, White Darkness, set on Haiti in 1915.

The Eighth Doctor would also find the TARDIS under danger first in the BBC book, The Bodysnatchers when two Zygons tried to enter the TARDIS in the Victorian period, and later in the Big Finish audio, The Girl Who Never Was. In this incident, his companion Charley Pollard was stranded in the far future and would be rescued by the Sixth Doctor when she uses her recall device.

The Doctor Who Magazine comic strip, The Crimson Hand, saw the HADS activate when invaders tried to arrest the Tenth Doctor and his friends; in retaliation, it rematerialised in a convenient part of the invaders’ ship so that the people it saved had the advantage over the aliens; and in the Eleventh Doctor novel, Dead of Winter, it’s explained that HADS can also allow the TARDIS to travel through time and space, leaving the Doctor, Amy, and Rory stranded in 19th Century Italy for several days before returning to them.

Finally in the recently released Doctor Who Magazine comic, Liberation of the Daleks, the Fourteenth Doctor finds himself stuck in a virtual reality with some Daleks. When the fake Daleks grow too intelligent and realise they aren’t real, they try to destroy everything around them. The TARDIS uses the HADS and the collapsing virtual reality to escape into the real world.

Despite being designed to save the TARDIS and its occupants, the HADS are very rarely used. Maybe it’s because the Doctor explained that he once spent three years orbiting Earth because he accidently left the HADS on that he doesn’t use them all the time.

Or maybe it’s because he’s worried of getting stranded somewhere: we know that the Doctor doesn’t like to hang about so perhaps instead of being a safety device, the HADS might actually be one of the Doctor’s worst nightmares?