The Doctor (David Tennant) takes Rose (Billie Piper) to a place called New Earth. In the year Five Billion and Twenty-Three, the population of Earth settled there after this planet’s destruction. The Doctor is summoned by the Face of Boe (Struan Rodger) to Ward 26 in a hospital in New New York, where incurable diseases have been cured by the humanoid feline nuns of the Sisters of Plenitude. Rose is separated from the Doctor, and the evil Lady Cassandra (Zoe Wanamaker)—a flat sheet on a rack—is transfered into Rose’s body—like a bouncy castle.
After Rose kisses him and shows knowledge of advanced computer systems, the Doctor becomes suspicious. In the hospital are thousands of artificially grown humans in pods. They are infected with every disease in the galaxy so they can be cured by the Sisters. Lady Cassandra knocks out the Doctor with a perfume gas and locks him in a pod. She then blackmails Sister Jatt (Adjor Andoh), demanding money to keep mum about their scheme. The Sisters refuse and Lady Cassandra releases several infected patients, including the Doctor. They in turn release others and a zombie-like horde attacks those who are not infected.
The Doctor and Lady Cassandra grab all the intravenous medical solutions and empty them into a disinfectant shower in a lift. This cures all the diseases. The Doctor encourages the cured to go out and spread the cure. The police arrest the Sisters, and the Face of Boe tells the Doctor he will have a message for him later. The Doctor orders Cassandra out of Rose’s body. Her servant Chip (Sean Gallagher) volunteers to accept her consciousness, but his body is cloned and begins to fail. Before she dies, the Doctor takes her back to see herself on the last night someone called her beautiful. That was Cassandra herself in Chip’s body, who dies in her own younger arms.
The episode was designed for Billie Piper to be funny, and as Lady Cassandra she was hilarious. The action takes place 23 years after the events of The End of the World. Adjor Andoh later played Francine Jones, mother of companion Martha Jones. Writer Russell T. Davis said that the clone pattern for Chip came from the earlier Cassandra’s encounter with Chip, thus creating a time-travel paradox. The hospital scenes were filmed in the Millennium Centre in Tardiff, which has a little shop of the sort this Doctor likes so much. It is called the Portmeirion Shop because it sells the unique Portmeirion China produced in the Welsh village that was used for the Doctor Who episode The Masque of Mandragora, and in The Prisoner with Patrick McGoohan in the Sixties. In the scene on present-day Earth, the graffiti of Bad Wolf is visible, much faded, on a wall. The voice Billie Piper used for Cassandra is her own natural accent. The difference between Rose and Cassandra’s Rose is the acting of Billie Piper and a Wonderbra.