Blon Fel-Fotch Passameer-Day Slitheen

Blon Fel-Fotch Passameer-Day Slitheen was a member of the criminal Raxacoricofallapatorian Slitheen family who took the skin of the human MI5 member Margaret Blaine.

Upbringing
According to the story she told the Ninth Doctor, Blon-Fel Fotch Passamer-Day Slitheen did not want to join in with her family's criminal activities, but knew that she would undergo unpleasant ritual execution with Venom Grubs if she refused. (TV: Boom Town)

Criminal career
By April 2006, the Slitheen family had killed Margaret Blaine so that Blon-Fel Fotch Passamer-Day could wear her skin. (TV: Aliens of London/World War Three) The Ninth Doctor believed she had done it personally. (TV: Boom Town, referencing TV: Aliens of London/World War Three) She went along with her family's plan to trigger a nuclear war on Earth, in order to devastate the planet and sell the radioactive remains as starship fuel. (TV: World War Three) and was present at 10 Downing Street with her fellow Slitheen, "Joseph Green" and "Oliver Charles". Green had taken over the government as Acting Prime Minister, which would give the Slitheen more power in Britain. Blaine was present when the Slitheen murdered General Asquith, and remained with Green for the rest of the crisis. She took the skins of fellow Slitheen members who had been summoned to Downing Street and also hunted the Ninth Doctor's companion Rose Tyler and future Prime Minister Harriet Jones. (TV: Aliens of London/World War Three)

When a missile fired by Mickey Smith struck Number 10 Downing Street, Blaine was the only Slitheen who managed to survive. (TV: Boom Town, referencing World War Three)

Blon escaped the destruction of 10 Downing Street by using an emergency teleport which only carried one so she was unable to save anyone else in her family. The teleport landed her in a skip on the Isle of Dogs before arriving in Cardiff, Wales where she managed to get herself elected Mayor. Secretly, she planned to take advantage of the space-time rift located there and advocated a plan for a proposed nuclear power plant called Blaidd Drwg. Secretly, she wanted to cause an "accident" at Blaidd Drwg. This disaster, located so near the rift, would destroy the Earth. This would provide her tribophysical waveform macro-kinetic extrapolator sufficient power for her to ride the "surf" into space to escape.

She killed anyone who questioned her plans, such as Mr. Cleaver. However, in a rare act of mercy, she spared the journalist Cathy Salt, after hearing the young woman talk about her family, reminding Blon of her own lost relatives. Soon after, the Ninth Doctor and his companions arrived to stop her. The Doctor remained uncertain as to what do, while Blon Fel-Fotch Passamer-Day pleaded the Doctor for mercy and clemency. When she tried to seize control of the Doctor's TARDIS by holding Rose hostage, the Doctor had her look into the heart of the TARDIS and she was regressed backwards along her timeline to the form of an egg. The Doctor returned her to her home planet to be placed with another family. (TV: Boom Town, Bad Wolf)

Other information
While she served as mayor of Cardiff, Idris Hopper served as her secretary. (TV: Boom Town, PROSE: The Twilight Streets)

Behind the scenes

 * Though, technically, Margaret Blaine has never appeared on Doctor Who, her Slitheen impersonator, Blon Fel-Fotch Passamer-Day Slitheen, gets credited as "Margaret Blaine" in the stories in which she appeared.
 * The infant Blon Fel-Fotch Passamer-Day Slitheen (played by an actor of appropriate size) would have appeared in the Shadow Proclamation in an earlier draft of The Stolen Earth. She would have been taken in by the Jingatheen family and, strangely, would be called Margaret by her father.