Doctor Who is a long-running British
science-fiction television series that began in 1963. It stars its protagonist,
The Doctor, an alien who travels through time and space in a ship known as the
TARDIS, as well as their travelling
companions. When the Doctor dies, they can undergo a process known as "
regeneration", completely changing the Doctor's appearance and personality. Throughout their travels, the Doctor often comes into conflict with various
alien species and antagonists. Cassandra (
Zoë Wanamaker) first appears in the 2005 episode "
The End of the World". A
transgender woman who hails from
Texas in the far future, Cassandra is driven by a sense of vanity, desiring beauty above all else, which is why she constantly undergoes surgeries. In "The End of the World", Cassandra is one of many guests at an event to view the destruction of the Earth by its expanding sun; secretly, she has also infiltrated the event with a number of robotic spiders, which sabotage the viewing platform. Intending to make it seem as though the event was a hostage situation so she could use the compensation money to fund further surgeries, Cassandra is forced to change her plans after the
Ninth Doctor (
Christopher Eccleston) reveals her scheme. Cassandra lowers the viewing platform's heat shields and teleports away, intending for those on-board to be killed by the sun's heat, but the Doctor is able to re-activate the shields. The Doctor teleports her back on-board, where she dries out and explodes due to the heat brought on from the sun while the shields were down. She subsequently appears in 2006's "
New Earth". Having repaired the damages to her body with new skin, Cassandra lay hiding in the basement of a hospital on the planet New Earth with her clone servant,
Chip (
Sean Gallagher), where she constantly re-watches a recording of the last party where she was called beautiful thousands of years prior. Discovering the
Tenth Doctor (
David Tennant) and his companion
Rose Tyler (
Billie Piper) on the planet, the latter of whom Cassandra had also previously met at the viewing platform, she lures Rose to the basement, where Cassandra uses a device to possess Rose's body. The Doctor and Cassandra discover the hospital's
Catkind nurses have been hiding human clones in the hospital's basement, the nurses infecting the clones with diseases in order to find cures. The Doctor figures out Cassandra has possessed Rose, and Cassandra frees the clones, resulting in a hospital-wide pandemic as the clones begin to infect others in a
zombie-like manner. She subsequently helps the Doctor cure the clones of their ailments to stop them from infecting both herself and others, constantly swapping between Rose and the Doctor's bodies. After the clones are cured, the Doctor forces her out of Rose's body and into Chip's body. Chip's clone body begins to die, and the Doctor and Rose take her back in time to the party Cassandra had been watching. Cassandra, in Chip's body, calls her past self beautiful before dying in her arms. Cassandra also appears in the short story
Lives of the Rich and Thin, published as part of the 2005 book
Monsters and Villains. The short story depicts her early life. == Conception and design ==