K9 Mark II

K9 Mark II was the second K9 known to the Fourth Doctor. Based on a robot originally built by Professor Marius, he was apparently constructed by the Doctor himself. (DW: 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 in E-Space in order to assist witht he liberation of the Tharil species. When Romana returned into N-Space, K9 joined her on Gallifrey. Once on the Time Lord's homeworld, he continued to serve Romana as she ascended to the post of Lady President of the High Council. (BFG: Weapon of Choice, et al)

Whilst on Gallifrey, he frequently liaised with K9 Mark I, Leela's version of the robot, whom he found to be "inferior". (BFG: Square One)

Biography
Mark II was built for the Fourth Doctor after Mark I decided to stay on Gallifrey with Leela. The Doctor appeared to have prepared for the possibility of losing Mark I, as he already had Mark II (or at least its components) prepared and in storage. (DW: The Invasion of Time)

Quest for the Key to Time
This version of K9 accompanied the Doctor and Romana 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. (DW: The Ribos Operation)

K9 killed pirates who were attacking other planets from their own, Zanak. (DW: The Pirate Planet)

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. (DW: 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. (DW: 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 supercomputer 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. (DW: 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 battled the Daleks and Davros. (DW: Destiny of the Daleks) Later he remained patiently in the TARDIS, which would imply that his voice had not yet returned. (DW: City of Death)

After his voice returned, K9 sounded different for some time. 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. (DW: The Creature from the Pit). Some time later, K9's original voice returned. (DW: The Leisure Hive)

In E-space, 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. (DW: State of Decay)

Damaged by the time winds, K9 Mark II could not cross back into N-Space with the Doctor, and decided to stay at the Gateway with Romana, to help her free the remaining Tharils from slavery. (DW: Warriors' Gate) The Doctor chose not to replace K9 again, though he did build/obtain a Mark III that he gifted to Sarah Jane Smith (KAC: A Girl's Best Friend) and apparently at one point also created a Mark IV to give to Sarah Jane upon the destruction of Mark III. (DW: School Reunion)

Mark II was repaired. He accompanied Romana when she returned to N-Space. During Romana's term as President of Gallifrey, K9 Mark II served as her aide and advisor. (NA: Lungbarrow) He may have also shared an adventure with Romana and the Eighth Doctor at some point after this (WC: Shada)

Personality
K9 was faithful to the Doctor and called him 'Master.' He corrected the Doctor when necessary. (DW: The Ribos Operation) He was happy to have a conversation with the supercomputer Mentalis, declaring it better than talking to unpredictable life forms. K9 branded optimism illogical, bordering on insanity. (DW: The Armageddon Factor) He declared that he didn’t feel anything nor did he find it necessary to do so. (DW: Warriors' Gate)

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

Features
After K9 Mark I was left behind with Leela on Gallifrey, The Doctor built an improved K9 Mark II. While the cosmetic changes were minimal (including the new metallic charcoal paint-work), in terms of series continuity K9's character underwent a vast change. 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 One model, the Doctor's own memory and knowledge, and that of the TARDIS. (DW: The Invasion of Time) Unlike Mark I, Mark II could climb stairs. (PDA: Heart of TARDIS)

Behind the scenes

 * One of the reasons the production team replaced K9 Mark I was because it was difficult to control. They overcame this problem when they built K9 Mark II.

Production notes

 * The production team used the same prop for Mark II as they had for K9 Mark I.
 * In WC: Shada, K9 mentions that Earth's soil interfered with his traction (a in-joke referring to the original prop's real-life difficulty travelling in a straight line).

Casting
John Leeson provided K9 Mark II's voice in almost all appearances. Another actor, David Brierley voiced him in The Creature from the Pit, Nightmare of Eden and The Horns of Nimon. Leeson took over the role again from The Leisure Hive onward.