The Doctor's TARDIS is stuck as a police box. Has that always been so? Specifically:
- has it had that shape right from the start
- has it appeared as a police box in every episode?
Technically not always, in "An Unearthly Child" we are introduced to the TARDIS and it appears it is the first time the chameleon circuit has failed. The Doctor says:
It's still a police box. Why hasn't it changed? Dear, dear, how very disturbing.
This establishes that the TARDIS is now stuck as a police box, but Susan reveals that it wasn't always a police box:
SUSAN: It should have changed. Wonder why it hasn't happened this time.
BARBARA: The ship, you mean?
SUSAN: Yes, it's been an Ionic column and a sedan chair.
BARBARA: Disguising itself wherever it goes.
SUSAN: Yes, that's right. But it hasn't happened this time. I wonder why not.
DOCTOR: This looks familiar.
PERI: Where are we?
DOCTOR: Scrap yard.
PERI: I didn't mean that. I meant, whereabouts on Earth are we?
PERI: It didn't change.
PERI: I thought you said it was going to blend into its surroundings.
DOCTOR: Oh, she's probably thinking about it. Come on, let's find out where those signals are coming from.
(The Tardis changes into a highly ornamented dresser.)
PERI: Oh, neat, Doctor.
PERI: Very neat. I mean, there's nothing at all incongruous about that.
DOCTOR: Well, she hasn't done it for a long time. She's out of practice.
PERI: Of course, Doctor.
Here is the TARDIS shown in mid-transformation and as the final dresser.
Later it transforms into a pipe organ (as pictured) and the Doctor plays it briefly.
(The Tardis materialises in Lytton's junk yard as a small pedal pipe organ.)
DOCTOR: This is getting ridiculous.
PERI: I'm not saying a word.
DOCTOR: I was certain I'd fixed it.
(He pulls out a stop and plays the opening line from JS Bach's Toccata and Fugue in B Minor.)
And lastly it turns into a gate.
(A pair of large wrought iron gates, the sort you'd find at the entrance to the driveway of a very large house, materialise in a corridor. The Cybermen lead their captives out.)
GRIFFITHS: What happened to the organ?
PERI: Do you really want to know? Oh, this place is so cold!
The TARDIS is also shown undisguised in "The Name of the Doctor" silver-grey cylinder.
This varies slightly from the undisguised TARDISes shown in "The War Games" serial.
The TARDIS has appeared as a police box since the first Doctor.
The Sixth Doctor made some progress towards fixing the chameleon circuit briefly, but the forms chosen did not blend in to the surroundings very well, and eventually he reverted it to the police box form.
There was also an audio short story entitled The Deep which dealt with Nyssa attempting to fix the chameleon circuit, and succeeding insofar as the TARDIS then took on the form of a whale, both inside and out. The Doctor undid Nyssa's repairs in order to make the TARDIS operable.
The TARDIS has been "stuck" as a police box since the very first episode - where it made sense, as the Doctor was hiding out on Earth and it was inside a junkyard.
There was only one story, during the Colin Baker era (Attack of the Cybermen), when he temporarily repaired the circuit. It still didn't work very well, and at the end he disabled it again as he'd become attached to the police box shape.
No not always, I've seen shows in the original series (before the renewed cannon of 2000's) where the doctor's TARDIS changed into several other objects in attempt to blend in.
Frankly the objects the circuit always chose did not always blend-in probably.
Eventually I think the Doctor got sick of it and it officially "broke" into the lovely blue box we all now. The doctor also said he never bothered to get it fixed because he loved the blue box when rose first asked about it in the newer series.
There are a number of episodes since 1963 that address the TARDIS' look. whilst it was almost always a police box, the TARDIS has been shown as other objects such as in the recent episode showing the Doctor stealing it from Gallifrey looking like a silver cylinder, and I can remember another classic episode where the doctor changed it into an old clock or oven or something to blend in with a rubbish tip after briefly fixing the circuit. However the TARDIS only changed shape after landing and the doctor exiting, which then lead to him taking a while to actually find the door when he got back, i think this was one of the reasons he kept it a police box as well as getting attached. and for BBC to market the spacecraft as merchandise (you can't market something that changes every episode!) The Master's TARDIS is also seen on various occasions as other objects such as a grandfather clock, as well as the Rani's TARDIS - both of which are completely different inside and out compared to each other and the Doctor's. It did appear in the first episode as a police box and i believe it certainly will in the last :(
The Doctor slightly addresses it in The Caves of Doom, but then, it’s not fully addressed until the story The Meddling Monk where it is called the camouflage unit. Later, in Logopolis the doctor first called it a chameleon circuit and also stated it was in repairs when he got it.
Also, the doctor’s TARDIS was old when he got it and although newer TARDISes have a switch under the console that pops a keyboard out that the pilot can enter a code to make it appear as anything they want, the older models use scanners to try and blend in and they broke easily. It has been referenced many times in the canon show and in comics and audio adventures things the TARDIS had become before the events of An Unearthly Child where it didn’t change for the first time.
The Doctor has attempted to fix it multiple times like in Attack of the Cyberman, and audio adventure where Nyssa tried and turned the TARDIS into a whale, the seventh doctor using his TARDIS in another timeline where he died in his third body and his chameleon circuit in PROSE. In an Eleventh doctor deliberately went back and destroyed the chameleon circuit in the comic Hunters of the Burning Stone. You can read more about it on the wiki.