The question at the heart of "The End of the World" is "What happens next?" This not only permeates the episode and colors the actions of its characters, but also inevitably rests in the mind of the viewer who, having been reasonably pleased with "Rose," now wants to know whether this series will continue to captivate. To answer that question, writer Russell T. Davies weaves a tale that emphasizes death, loss, and the challenges of moving on, all centered around the planned demolition of planet Earth five billion years from now. Many seeds are planted in this episode, including the first mention of the "Bad Wolf" and the first appearance of the Face of Boe, but at its core, this is a story of three very different people and how they deal with endings.
The demolition of Earth is not a grand tragedy. It is in fact a planned, controlled, perfectly safe event motivated largely by finance and viewed from a nearby satellite by a gathering of spectators, including the Doctor and Rose. It's no different than knocking down a building. No lives are lost either, as the planet had been emptied long ago. Humanity has scattered among the stars, interbreeding with various alien species and integrating with the fabric of the universe. Even the trees have evolved, achieving sentience and leaving Earth behind for planets such as Cheem. What is the Earth, then, but an empty shell of its former glory, a habitat with no inhabitants? And what does the end of the world matter if its people have long since moved on?
"Moving on" is central here, and the demolition is significant in that it draws out how each of three characters handles doing so. The first is Cassandra. Surgically altered over the course of centuries, Cassandra is a sheet of skin with something vaguely resembling a face at its center. She is also a xenophobe, clinging bitterly to the notion that she herself is the only human remaining in existence, and that mankind's descendants have become impure by intermingling with species from other worlds. Cassandra is, of course, the villainess of the episode - we aren't yet at the point in the series where such traits can be applied to a potentially heroic figure - but in many ways she is reacting naturally to the sweeping change of the centuries: by denying it. How many of us, with our own paradigms of what is, isn't, and should be, have ever ignored the realities of events that threaten to intrude upon our carefully constructed worldview? Cassandra believed that humanity was one particular thing, and when that thing changed, she refused to change with it. When her belief structure reached its logical end, she could not let go. She clung to it, demanding that it survive through her, because she couldn't face the world without it. She couldn't change, she couldn't adapt, and because it, she allowed herself to become a monstrosity that parodied "pure" humanity. She didn't want to know what happened after "the end." She didn't even acknowledge the end when it came.
Circumstances place Rose in opposition to Cassandra, both narratively and philosophically. Over the course of the episode, Rose - genuinely the last of the humans - comes to understand what happens after "the end," although she can't quite accept that the end must come. When the planet is finally demolished, she can only mourn it. Rose, a woman of our time, says to the Doctor, "We were too busy saving ourselves, no one saw it go." But when ever the trees have moved on, what's left to hold onto but the memory of what once was? Rose can't let go of the past in that sense, because she can't understand that everything eventually has to end. But unlike Cassandra, Rose moves on. She makes herself better and gets on to the next thing. She accepts it. In many ways, Doctor Who is a series about humanity's grand potential, and in Rose's ability to move on to new things, we see that potential made manifest. It's the same thing that enables the peoples of Earth to reach the stars, and it's of the things that the Doctor admires about humanity.
The Doctor himself pretends to be able to move on. When Rose questions him about his past, he refuses to tell her anything. He tries to make it seem that his vision is fixed firmly on the future. But the Doctor understands endings. "Everything has its time and everything dies," he says as he watches Cassandra's gruesome death (or would-be death - more on that several episodes later). But he doesn't understand moving on. He tries to block his past out, but instead can only relive it.
The Doctor is the survivor of a war. He belongs to a race of people called the Time Lords, and he is the only one left. His planet burned. And now he is alone. He believes in consequences, and has no qualms about letting Cassandra die for her actions. He endangers innocent lives just as he saves them, and he inspires others to give their own to save his. Jabe of Cheem becomes the first in a long line of one-off characters to sacrifice herself to allow the Doctor to save others. He recruits Jabe to come with him at the end of his mission, and it is difficult to see what other purpose he would have for doing so. He seems to expect it. Is this a consequence of the war, or is this the core of the man?
And above all, the Doctor is lonely. Rose learned to move on to the next thing in life, but the Doctor is perpetually looking back at the war that left him alone, and at least subconsciously, he wanted Rose to know how he felt. And when he finally opens up at the end of the episode, we get another glimpse of who the Doctor is. There is a reason that the Doctor brought Rose to the point in time where her planet dies by fire - so that she can experience his pain without having to live it. He wanted her to see her planet burn, to be the last of her kind, to contemplate the end of her world, and perhaps to show him how to move on from it. But what kind of man exposes an innocent girl to that horror? What kind of man seeks to relieve himself of his own suffering by causing the same suffering in someone else, someone he professes to care for? As Rose notes early in the episode, she doesn't know the Doctor or anything about him. Is he the kind of man genuinely worth knowing? Is the good that he does worth the darkness in his soul? Again, this is the central paradox of the Doctor's existence - he performs grand universal feats of good, and destroys individual lives to do it.
And then goes and eats chips with his friend.
"The End of the World" says that endings are not in fact the end. Rose learns the lesson. The Doctor will learn it eventually, but he'll do so at a terrible cost.