In episode 2 of season 2 of Doctor Who, Rose and the Doctor are off to New Earth in the year 5 Billion. Earth was recently destroyed by the Sun's supernova (As seen in season 1 episode 2, The End of the World). Yet, this has led to a renaissance and the founding of New Earth, with its capital city New New York.
There are three story lines in this episode, all of which deal with the concept of death and letting things pass when their time has come.
First, the Doctor is called to the hospital by a patient, the face of Boe, perhaps the oldest being in the universe whom we also met in "The End of the World." The Face of Boe is just that, a giant face who exists in a glass cylinder. In a conversation with the Face of Boe's nurse, we hear legends of the Face of Boe, that he is thousands, if not millions of years old, and that there is a legend that at the end of his life he will impart a secret to one like himself, a wanderer, a man without a home, a lonely god. Is the nurse referring to the Doctor? The Face of Boe appears to be dying. Has his time come? Will he reveal a secret to the Doctor?
The second story is that of the last human being, Cassandra (also encountered in "The End of the World") trying to take over the body of Rose. Cassandra is a grotesque parody of a human. Through plastic surgery, she exists, also as only two eyes and a mouth in the middle of a stretched piece of skin. Her constant call to her attendant is: "moisturize me." The Face of Boe and Cassandra form interesting foils. The face of Boe is truly ancient, and is letting himself die. Cassandra seeks to continue in existence through unnatural means and will not let herself die. In Rose, a human as they should be, Cassandra sees a way to lengthen her life. Through a psychic transfer, Cassandra is able to place her consciousness in the body of Rose, giving her a new lease on life. This story line is about Cassandra realizing, with the help of the Doctor, that there is a time for all things to come to an end. Cassandra meets her end in this episode, but it is with hope and redemption, not despair.
Third, the Cat-Nurses of the hospital have bred human flesh, infected with all human diseases, as a means of curing human illnesses. The nurses, through the use of these human clones bred only to be inflicted with human diseases, are able to cure all of these diseases for humanity. Theirs is pure utilitarian thinking. The good of the many through the suffering of a few (although there seem to be thousands of these human clones). Here the Doctor appears as the utmost authority, condescendingly condemning the nurses. He says that if they seek any higher authority, there is none.
In the end, the Doctor, fully living up to his name, becomes the healer of all, healing all diseases and granting new life, in essence, creating a new human subspecies to which he has given life. Their time had not yet come, so the Doctor grants life from death.
The Doctor's actions inspire the Face of Boe. Though he thought his time had come, he now sees that it has not. He now seeks to continue living, not in a selfish manner like Cassandra, but because he sees that he still has good to do in this world.