O is for... Osgood (2)

Petronella Osgood became UNIT's Scientific Adviser shortly before a bizarre incident took place at the National Gallery in London. A number of paintings appeared to have had their glass broken from the inside, and several marble statues had been smashed. 

Osgood was a big fan of the Doctor and often wore items of clothing based on his own tastes. On meeting the Doctor at Trafalgar Square she was wearing a scarf similar to the one he had worn in his Fourth incarnation. Left alone with a colleague to investigate the Gallery, Osgood was attacked by Zygons. It transpired that thousands of Zygon refugees had arrived on Earth in Elizabethan times, and had hidden themselves within stasis cubes - used by some races such as the Time Lords to create living three-dimensional art, in which the viewer could physically immerse themselves.

The Zygons had now emerged into the Gallery, with some of their number hiding under sheets, after having first smashed the statues which they were covering.

One of the Zygons then took on her form, but Osgood managed to escape. In their efforts to forge a peaceful alliance between UNIT and Zygon, a trio of Doctors used an amnesiac gas to make both sides forget who they were - human or alien. Osgood was the only person able to know as she required an inhaler for her asthma. As part of the peace process, they elected not to tell anyone which was which.

Osgood remained on the Zygon repatriation scheme, with her duplicate and together they devised the "Osgood Boxes". One was said to contain a virus which would destroy every living Zygon - developed by Harry Sullivan at Porton Down - whilst the other would expose all the disguised Zygons, as the scheme agreed with UNIT involved the aliens living incognito amongst humans.

Some younger sections of Zygon society resented their having to live hidden lives, and a militant group wanted to simply reveal themselves and take over.

The Doctor and Clara were reunited with Osgood when UNIT laid a trap for Missy at St Paul's Cathedral in London. The female incarnation of the Master was working with the Cybermen to unleash an army across the Earth. Missy was captured, but whilst being transported on a UNIT aircraft managed to escape her bonds. She killed Osgood - but it was never known of this was the original human one or the Zygon duplicate.

When the young Zygon militants finally began to take action against their elders, Osgood was abducted by them. Their leader - Bonnie, who had taken on Clara's form - wished to gain control over the Boxes - intending to use the one that would eliminate their human identities. 

The Doctor rescued her, and at one point the pair had to parachute from a crashing aircraft, shot down by Bonnie.

The Doctor refused to say which Box was which, so that Bonnie would be forced to choose. It transpired that the Boxes were fakes - designed simply to maintain the peace as there was an equal chance of killing them all as of exposing their true identities. Realising the futility of her actions, Bonnie elected to take on Osgood's appearance, so that once again there appeared to be two of them - and still no-one would ever know if they were both Zygon, or human and Zygon. 

When the Grand Serpent attempted to assassinate Kate Stewart after taking over UNIT, it was Osgood whom she called on for help.

Played by: Ingrid Oliver. Appearances: The Day of the Doctor (2013), Dark Water / Death in Heaven (2014), Invasion of the Zygons / Inversion of the Zygons (2015).

• Oliver auditioned for the role wearing her boyfriend's thick-rimmed spectacles. She was asked to retain them as they fitted the "nerdy" nature of the character.
• As mentioned under "O is for... Osgood (1)", Steven Moffat intended that she would be the daughter of Sergeant Osgood from The Daemons, though he chose not to make this explicit - but spin-off literature has messed this up.
• Oliver first came to fame as one half of the comedy duo Watson & Oliver. The pair had met at school.
• She's the partner of the host of Richard Osman's House of Games. They met when she guested on his celebrity game show.