The TARDIS is an amazing machine. I’m not talking about it being a time machine or a spaceship, or how it’s bigger on the inside, or how it was constructed (not saying how – spoilers!), or what the Doctor thinks about it (more spoilers!). I’m talking about the TARDIS as a literary device. Let’s take a look at it.
The Doctor’s TARDIS is unique. TARDISs in general work the same way, except that their Chameleon Circuits are working, so they can actually change appearance. The idea is that before they land, they take a survey of the surroundings and select an appearance that would blend in, so that the TARDIS isn’t noticed by the local inhabitants. I’m not quite sure how well this worked in practice; in the city of Logopolis (in the episode “Logopolis”), the entire city was made of round stone huts, but the Master’s TARDIS chose the form of a Greek column. The locals didn’t notice this, but still, it really didn’t blend in at all. But no matter: the Doctor’s TARDIS’ Chameleon Circuit doesn’t work and it’s stuck in the form of a blue police box.
So there you have it. The TARDIS is in a form that people today wouldn’t even recognize if it wasn’t for the Doctor Who show itself (and of course, the inhabitants of the Doctor Who universe do not have Doctor Who on television, so they don’t recognize it at all [though, I suppose they could have Doctor Who on television, which would lead to a lot more nested brackets in this sentence]). It also seems to have a perception filter on it, so that people who don’t know what the TARDIS is tend to not see it. At least, that’s the only explanation I could come up with for why people aren’t always staring at it. Some people are able to see it without a specific reason to (a boy spray paints graffiti on the side of it in one episode, for example).
My estimate is that on the outside, the TARDIS is a little less than two meters square (yes, I’m American, but the show is British and the metric system is superior anyway) and perhaps three meters tall. On the inside, though we see very little beyond the console room in the new series, the TARDIS has a vast interior. The Eleventh Doctor hints at the size of the TARDIS in his first episode when he mentions that when TARDIS landed on its side, the swimming pool was now in the library, but consider this. In “Castrovalva,” just after the Doctor regenerated into his fifth incarnation, he unravels the Fourth Doctor’s scarf, tying one end to the door of the console room, so that he can find the Zero Room using the yarn as a path back. He eventually runs out of scarf and has to start dropping clothing. Considering that the scarf was around 18 feet long and used at least a thousand yards of wool (I’ve knitted a Fourth Doctor scarf and I can tell you, this estimate is very low), the TARDIS is extremely huge. (Due to the events in the episode, the Doctor jettisons 25% of the TARDIS’ volume, but it’s still huge.)
The TARDIS is not only capable of traveling in time, but can travel like a normal spaceship through space, with enough power to tow a spaceship away from a black hole (in “The Satan Pit”). Considering the size of the TARDIS, this is pretty incredible: on this chart of spaceship sizes, the TARDIS is about a half a pixel big (if that), towing something that’s at least ten times its size (probably more), out of an anomaly from which nothing else can escape. However, normally, the TARDIS doesn’t travel like that. It dematerializes and then materializes at its destination, allowing it to appear anywhere it can fit. It can materialize on top of another object, either assuming its shape (as it did in “Logopolis,” when it landed on a real police box) or take it inside itself (it appears around the Doctor and Donna Noble in “The Runaway Bride”).
From a narrative standpoint, the TARDIS is designed to give as much freedom to the story as possible, and I’m not just talking about being able to take the Doctor and his companions to any point in space and time. It’s extremely small, so it can appear in almost any space (such as Martha Jones’ very cramped student apartment), and yet its interior is vast enough to supply any room needed. It doesn’t require landing space or time-expensive landing sequences; to achieve the same effect, the designers of Star Trek invented transporters so that the crew could travel to and from the Enterprise without traveling in shuttles all the time. The writers don’t have to sacrifice valuable screen time to show how the Doctor gets to where he needs to be. This is especially true for the new series, in which episodes are usually 45 minutes long, instead of the two hours the classic series had.
It was a stroke of genius to break the Chameleon Circuit and lock the TARDIS’s shape. The blue box and its characteristic VWOOP-VWOOP-VWOOP sound are iconic. Other shows have their recognizable spaceships, but they all boil down to a silver tin can that moves through space with some kind of rocket-based propulsion. The TARDIS, however, is the TARDIS, not mistakable for anything else. Once you’ve seen one or two episodes, the sound of the TARDIS makes your heart skip a beat because it means that the Doctor’s on the scene. But, it’s even more important to the characters. The Doctor’s enemies panic at the sight of the blue box. To Jackie Tyler and Mickey Smith, the sound means Rose is coming home. To Wilfred Mott, it means that the Earth is about to be saved. And in “School Reunion,” the blue box warns Sarah Jane Smith of the return of the most important man in her life, someone she had met and not recognized only a few hours before. The Doctor may change, but the TARDIS stays the same. And, of course, the non-traditional method of travel adds to the Doctor’s air of mystery and eccentricity.
The TARDIS, as a symbol and as a literary device, is as important to the show as the Doctor himself. Would the show be as compelling without it? I would conjecture that since the show shines on its plot and dialogue, it wouldn’t suffer from having a traditional spaceship/time machine, but the presence of the TARDIS streamlines the plot by discarding the need to deal with the mode of transportation and provides an identifiable symbol that unifies the many faces of the Doctor.