Claustrophobic, quick-paced, character driven, and magic. David Whitaker is one of the strongest people ever involved with the show, and he rarely gets the credit he deserves. This story, like the first part of An Unearthly Child knows exactly what the TARDIS is. This is something about these early episodes that fascinates me, as El Sandifer points out in her excellent books, it's like the show already knows that rickety old police box is magic, humming with power, iconic. And here, the show finally tells us that this magic box is also alive. An exhilarating episode, and the first example of the most successful context in which Who can be written: "We ran out of time and money and need something now." All our best stories are made like that.