K9 Mark II

K9 Mark II was the second K9 known to the Fourth Doctor. He was stored in a box inside the Doctor's TARDIS by the time K9 Mark I stayed behind on Gallifrey. (TV: The Invasion of Time)

The unit was particularly associated with Romana, having served both Romana I and Romana II. Indeed, he stopped travelling with the Doctor at the same time as the second Romana, with whom he remained on Gallifrey whilst she pursued politcal endeavours. He continued to serve Romana as she ascended to the post of Lady President of the High Council.

Biography
After K9 Mark I decided to stay on Gallifrey with Leela, the Fourth Doctor already had Mark II (or at least its components) prepared and in storage in his TARDIS.

Quest for the Key to Time
This version of K9 accompanied the Doctor and Romana I on their quest to locate the segments of the Key to Time. The Doctor used a dog whistle to alert K9 and to summon him. (TV: The Ribos Operation)

K9 killed pirates who were attacking other planets from their own, Zanak. He also fought the Pirate Captain's robot bird. (TV: The Pirate Planet)

K9 helped apprehend the Cessair of Diplos and helped Romana find the Doctor on the Megara ship. (TV: The Stones of Blood)

On Tara, K9 disabled several guards in the employ of Count Grendel of Gracht. He travelled in a boat with the Doctor in a passage under the Count's castle, cutting a way through with his blaster. The Doctor left K9 in the boat as he went to foil the Count's plot. (TV: The Androids of Tara)

When the TARDIS landed on Delta III, the swampy terrain made it impossible for K9 to accompany the Doctor and Romana. He remained in the TARDIS. (TV: The Power of Kroll)

While searching for the sixth segment, they landed on the planet Atrios. K9 was tricked into entering a scrap metal furnace, but the Doctor rescued him before he was melted. K9 transmatted to the planet Zeos. K9 told the super-computer Mentalis it was good to talk with someone who was not as illogical or unpredictable as a flesh-and-blood being. K9 again transmatted, arriving on the third planet. He was turned by the Shadow to do his bidding, but the Doctor was able to remove the control matrix implanted by the Shadow, restoring him to his former self. (TV: The Armageddon Factor)

Further Adventures
While making repairs to K9's brain circuit, the Doctor found that K9 had contracted laryngitis and could not speak. K9 was left in the TARDIS while the Doctor and Romana II battled the Daleks and Davros. (TV: Destiny of the Daleks) After his voice returned, K9 sounded different for some time. (TV: The Creature from the Pit, Nightmare of Eden, The Horns of Nimon)

K9 remained patiently in the TARDIS while the Doctor and Romana visited Paris. (TV: City of Death)

On Chloris, K9 attacked some Wolfweeds and the soldiers of Lady Adrasta. The Wolfweeds swarmed about him and entwined him in fibres. He was taken by Adrasta, as metal was valuable. (TV: The Creature from the Pit)

When two ships, the Empress and the Hecate, collided in space and the Mandrels were released, K9 helped fight them off with laser beams. He also assisted in uncovering a drug smuggling conspiracy by Tryst. (TV: Nightmare of Eden)

K9 and the Doctor investigated the planet of Skonnos, where the Nimon were planning to invade. He helped stop them by navigating the Doctor through complex tunnels to escape and informed the Doctor of the history of the Skonnan Empire. Soldeed incapacitated him and send him off to examination. (TV: The Horns of Nimon)

K9 was left inside the TARDIS when the Doctor and Romana went punting in Cambridge to meet Professor Chronotis. After several hours of being "nowhere", (WC: Shada) due to the Doctor and Romana being taken out of time by Borusa, (TV: The Five Doctors) the Doctor went back to the TARDIS, demanding everyone to put their bathing suits on as they were going to Brighton. The Doctor and Romana, though retaining memories of the punting trip, had forgotten they intended to visit Chronotis. The Doctor said that they had intended to be back "before lights out", but "went on somewhere else", according to Romana, after only being away from the TARDIS for two hours and twenty-one minutes. (WC: Shada)

K9 was damaged at Sentry Castle after Anding threw him down a flight of stairs. (TV: The Castle of Doom) The Doctor and Romana were trying to repair him when they were caught in a time loop caused by Meglos. Upon being repaired, K9 tried to find the Doctor and Romana, only to shut down in the forests of Tigella. He was eventually able to help defeat Meglos. (TV: Meglos)

K9 led the attack on the castle of the Three Who Rule. He shot many of the guards, doing most of the work. The Doctor made Ivo apologise to K9 for doubting him. (TV: State of Decay)

K9 encountered Hadale alongside the Doctor and Romana and helped to clear the Doctor's name. He joined Hadale's quest to defeat Anepco and rescue Orina, and proved crucial in finding an alternative entry into Anepco's fortress. Whilst he was shot by one of Anepco's henchmen, he was later repaired. (TV: Dragons of Fear)

K9 helped the Doctor prove the existence of the outside universe to the people of Halley's comet, using his databanks to provide detailed factual information. During the battle between the natives and Captain Tork, K9 was reprogrammed by Torek's men to serve them, but was coaxed out his reprogramming by the Doctor, helping him and the defected Lieutenant Tela to create a bomb to destroy the control room. (TV: Into the Comet)

Once the TARDIS arrived on Gallifrey, K9 was the only person onboard the TARDIS who did not have multiple timelines of himself converge. He would play a vital role in uncovering Supreme Treasurer Halvak as having been behind all the goings-on, and opted to stay with Romana when she took up Halvak's former position. (TV: Sealed Orders)

Personality
K9 was faithful to the Doctor and called him "Master" — calling him "Doctor" only once, and this was when under the control of the Shadow. (TV: The Armageddon Factor) He corrected the Doctor when necessary. (TV: The Ribos Operation) He was happy at the prospect of going on holiday (TV: The Ribos Operation) and to have a conversation with the super-computer Mentalis, declaring it better than talking to unpredictable life forms. K9 branded optimism illogical, bordering on insanity. (TV: The Armageddon Factor) He likewise branded the claims of the High Priest Levish as being "illogical, and insufficient as claims on the understanding of the wider universe". (TV: Into the Comet)

K9 had no problem with killing those who threatened him, such as Count Grendel's men or the guards of the Three Who Rule. (TV: The Androids of Tara, State of Decay) He only killed in self-defence, as his programming dictated. (TV: The Creature from the Pit)

The Doctor and K9 were fond of each other, as the Doctor described K9 as his "best friend" and when they were due to be destroyed in the TARDIS — which they actually survived — the Doctor placed a first place ribbon onto K9, and K9 thanked him.

The Doctor said that K9 had saved his life lots of times. (TV: Nightmare of Eden)

Features
After K9 Mark I was left behind with Leela on Gallifrey, the Doctor built an improved K9 Mark II. Gone was the product of Marius' 5000 AD technology. This new K9 was a product of the Doctor's own technology and values. The gun was now only capable of stunning — not killing — and K9 now responded to a dog-whistle. K9 also featured a vast memory-bank, seemingly a mixture of the Mark I model, the Doctor's own memory and knowledge, and that of the TARDIS. (TV: The Invasion of Time) He also contained sound-recording and surveillance camera software, (TV: Sealed Orders) and a blowtorch that could cut through metal. (TV: Dragons of Fear)

Casting
John Leeson provided K9 Mark II's voice in Season 16 and part of Season 17. Another actor, David Brierley voiced him from The Creature from the Pit onwards, while Roy Skelton, better known as a Dalek voice actor, provided the character's voice in Destiny of the Daleks when all was required was a croaking sound.

Season 16

 * The Ribos Operation
 * The Pirate Planet
 * The Stones of Blood
 * The Androids of Tara
 * The Power of Kroll
 * The Armageddon Factor

Season 17

 * Destiny of the Daleks
 * City of Death
 * The Creature from the Pit
 * Nightmare of Eden
 * The Horns of Nimon
 * Shada

Season 18

 * The Castle of Doom
 * Meglos
 * State of Decay
 * Dragons of Fear
 * Into the Comet
 * Sealed Orders