The Doctor takes Rose Tyler to New Earth, the planet where humans live after the original Earth is no longer. He takes her to a hospital which is sending out a signal for The Doctor to follow. Inside are the sick who are looked after by cat-like Sisters of Plenitude nuns. The sisters are able to cure nearly every virus/disease or condition that inflicts the human race. When they are separated, they enter the lifts. The Doctor goes up whereas Rose goes down to the basement.
In the basement is their old enemy, Cassandra from the last series. She has managed to find a way of transposing her soul from her sheet of skin to a person. She moves into the body of Rose intending to stay and enjoy the body. The Doctor heads off to search for Rose and discovers the sisters truth. They have people in glass containers with every imaginable disease possible. In a melee between the Doctor and Cassandra, the diseased people are released. The Doctor has to find a way of resolving the situation whilst also dealing with Cassandra`s soul. In the end, he puts a variety of cures into the air conditioning system to cure the ill. During the episode, he sees the Face of Boe who at the end says the next time they meet will be the last.