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, that when she was thirteen Blon-Fel Fotch Passamer-Day Slitheen took part in her first hunt and knew that if she did not join in with her family's criminal activities, she would undergo unpleasant ritual execution with Venom Grubs by her father if she refused. (TV: Boom Town)

Criminal career
By March 2006, the Slitheen family had killed Margaret Blaine so that Blon Fel-Fotch Passamer-Day could wear her skin. (TV: Aliens of London) The Ninth Doctor believed she had done it personally. (TV: Boom Town)

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. When Rose Tyler, Harriet Jones and Indra Ganesh discovered the body of the Prime Minister, Blon taunted them and started to kill Indra. (TV: Aliens of London)

Blon hunted down Rose and Harriet, but was distracted by the Doctor when he entered with a fire extinguisher. The Doctor, Rose and Harriet placed themselves safely behind the walls of the Cabinet. Thinking they couldn't do anything trapped inside the "box", Blon continued with the plot. She took the skins of fellow Slitheen members who had been summoned to Downing Street and waited for the launch codes to be given to Downing Street. The Doctor was able to contact Mickey Smith to launch the missile, UGM-84A at Downing Street with him still inside, while the Doctor, Rose and Harriet rode out the explosion inside a small, sturdy cupboard. (TV: World War Three)

Blon escaped the destruction of 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, 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 knew that failing this, anyone who arrested her would take the extrapolator for themselves, having "considerable technology of their own" and being "captivated" by the extrapolator. The extrapolator was programmed to lock onto the nearest alien power source and open the rift, still allowing her 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; Jack Harkness confiscated the extrapolator and used it to refuel the TARDIS quicker. The Doctor remained uncertain as to what do as she was sentenced to death on her homeworld in absence; Blon pleaded the Doctor for mercy and clemency. When the extrapolator began opening the rift through the TARDIS, Blon, threatening Rose, demanded Jack to place the extrapolator at her feet. After the extrapolator nearly pulled part the TARDIS, the heart of the TARDIS was forced open by and Blon looked directly into its light; 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)

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.