M is for... Midnight Entity

A creature of obscure origins which inhabited the planet Midnight. It was believed that no life could possibly exist here due to it being flooded by X-tonic radiation which vapourised organic matter.

Visiting the planet, the Doctor took part in a cross country tour in an armoured vehicle - the Crusader 50. It broke down after taking a detour from its planned route and came under attack just after its navigator-engineer thought he had spotted movement amongst the crystal rocks nearby. Some unseen being attempted to break its way. Shortly after it stopped, it seemed to have succeeded in entering and possessing the body of one of the passengers - a woman named Sky Silvestry. 

In its host body it began to assimilate the speech of the others, repeating everything they said. It seemed to latch on to the Doctor who had tried to befriend Sky, and it was later noted that she spoke at the same instant he did - before then speaking his words before he had uttered them. The Doctor found himself incapacitated physically and mentally. Believing that the entity had transferred into him, Sky led the others in trying to throw the Doctor out of the vehicle into the deadly environment. 

Realising they were being manipulated by the creature, the tour guide sacrificed herself by opening the doors as she seized Sky - and the pair were sucked outside.

As it had previously existed on the surface, it is probable that it continued to survive the loss of its physical host.

Played by: Lesley Sharp (Sky Silvestry). Appearances: Midnight (2008).

• Sharp, one of Britain's most accomplished actors, claimed that she was inspired to take up the profession by watching The Dick Emery Show as a child.
• Her first film role was in Rita, Sue and Bob Too (1986), and she went on to appear in Ken Loach's Naked. A major TV role was in Clocking Off.
• RTD always intended to have her appear in Doctor Who after she had appeared in his The Second Coming, starring Christopher Eccleston.
• The exact nature of the entity is never revealed. It may have had a physical form, though the engineer might have simply imagined seeing movement. It could physically attack the Crusader 50, yet managed to enter the vehicle whilst still intact and possess a human host - suggesting a non-corporeal being.