Dogs have, of course, been man's best friend seemingly since time immemorial - but the Fourth Doctor & K-9 were so much more than "master" & "tin pet".
Sadly Professor Marius was unable to keep him at the conclusion of The Invisible Enemy, but having borrowed the handy metal pooch to help in the eventual defeat of the Swarm, it seems Leela wants to adopt him despite protestations from the eccentric bescarfed Henry Higgins to her Sevateem Eliza Doolittle.
DOCTOR: Goodbye.And so like many a parent faced with a child wanting a furry friend, the Doctor allows the metallic version into the TARDIS.
MARIUS: Goodbye, Doctor, and thank you for everything you've done for us.
DOCTOR: It's been a pleasure, Professor. It's been a pleasure. Do you know that without K9 I think we'd have been part of the Swarm by now.
LEELA: We'd never have managed without him. Her. It. Sorry, K9.
K9: Apologies are not necessary, but thank you, mistress.
MARIUS: I think K9 has taken to you. Oh. Actually, I have to return to Earth shortly and you could do me a great favour if..
LEELA: Take K9 with us?
LEELA: Yes! Oh, please, Doctor, please, please, let's take him.
Mark I would serve with distinction following a quick repair job in Image Of The Fendahl up until The Invasion Of Time.
Faithful to the end, he stays behind on Gallifrey to take care of his ''mistress''- leaving behind an excellent disciplinary record of not chewing the furniture or befouling the floor of the console room.
DOCTOR: Well, ta-ta, everybody. Come on, Leela. Come on. What's the matter?Despite the initial reluctance to take the tin dog aboard, upon his leaving a new model is constructed- Mark II. Might the Doctor be warming to his new role as dog-owner after all? Of course he can't resist taking any requests or orders almost completely literally, mind. Though his mechanical mind computes so much more than "sit", "stay", "fetch" & bringing/attempting to hump the Doctor's slippers. If validation of his mettle as a companion were needed, consider his attempts to help Romana acclimatise to Earth idioms in The Stones Of Blood.
LEELA: I'm staying.
DOCTOR: What? Staying? Here? Why? Ah, I see.
ANDRED: I hope that...
DOCTOR: Yes, I'm sure you do hope. She'll look after you. She's terribly good with a knife. Come on, K9.
K9: Negative. I remain.
K9: To look after the mistress.
LEELA: I will miss you.
What he subsequently forgot with regard to tennis, he would make up for with a variety of other factual titbits on a wide range of subjects as well as serving as a perfect foil to the man who'd made him- the straight man to the Doctor's chirpy eccentric. "Optimism: belief that everything will work out well. Irrational, bordering on insane."
But isn't that what made his seven years as holder of the TARDIS key what they were? And as his relations with Romana changed following her regeneration into a younger and comparatively more carefree new incarnation, K-9 would undergo a change of his own following a bout of laryngitis - long-time voice actor John Leeson taking his dulcet tones elsewhere following The Armageddon Factor , to be replaced with David Brierley in the nick of time for The Creature From The Pit.
His bout of illness lasted only one season before Leeson was tempted to return. And so he was at full power once more for Season 18, something of a last hurrah for both he & the man he had beaten at chess not once but twice.......until a very special cameo for the know it all canine, alongside a very different Doctor. The RSPCA would surely not approve of such neglect, but no matter. Mickey Smith of course knew how he felt, having come to the crushing realisation that he had taken the place of the tin dog who would serve as something of a back-seat driver before sacrificing himself to ward off the Krillitane.
Maybe he's still up there in space somewhere, too - NASA wheeling out a robot named in his honour to help in studies of Mars. As SF Gate reported of a demonstration of its abilities:
"K-9, with its six ribbed wheels and its gangly mast obviously modelled after the twin Mars Rovers Spirit and Opportunity, used its own built-in intelligence to wheel around obstacles in order to photograph Broccoli. It obeyed generalized commands from computer-wielding technicians placed inside a simulated nearby habitat."Oh yes. And the Doctor clearly never forgot him even half a dozen regenerations on from the man who'd first found the dog foisted upon him.
DOCTOR: K9! Rose Tyler, Mickey Smith, allow me to introduce K9. well, K9 Mark Three to be precise.Having been helped back onto his feet, metaphorically at least, he does something incredibly out of character- becoming an action hero! Perhaps inevitably one of his best known lines becomes his statement of intent as he prepares himself to blow the nasty humanoid bats to Kingdom Come.
ROSE: Why does he look so disco?
DOCTOR: Oi! Listen, in the year five thousand, this was cutting edge. What's happened to him?
SARAH: Oh, one day, he just, nothing.
DOCTOR: Well, didn't you try and get him repaired?
SARAH: Well, it's not like getting parts for a Mini Metro, Beside, the technology inside him could rewrite human science. I couldn't show him to anyone.
DOCTOR: Ooh, what's the nasty lady done to you, eh?
FINCH: You bad dog.
"It's all right. He was just a daft metal dog. It's fine, really."Every dog has his day, and one final act of kindness before departing - this time having said a proper goodbye to make up for the last - from the Doctor allows K-9 to have many more.
SARAH: Well, there was this one guy. I travelled with him for a while, but he was a tough act to follow. Goodbye, Doctor.One seemingly permanently sleeping dog is given a helping hand towards continued existence!
DOCTOR: Oh, it's not goodbye.
SARAH: Do say it. Please. This time. Say it.
SARAH: But you were blown up.
K9: The Master rebuilt me. My systems are much improved with new undetectable hyperlink facilities.
SARAH: Oh, he replaced you with a brand new model.
SARAH: Yeah, he does that. Come on, you. Home. We've got work to do.