Ghosts, Angels, Zombies
Roughly humanoid gas creatures
“The Unquiet Dead”
Anything You Can Do, Shortness of Breath
An incorporeal life-form, the Gelth represented a personal tragedy for The Doctor, as it was implied that they only became the threat that they were as a result of the damage caused in the crossfire of the Time War.
Apparently a formerly corporeal species, the Gelth were apparently caught in the crossfire during the Time War between the Time Lords and the Daleks, resulting in their bodies being destroyed, although they themselves managed to survive to some degree through unspecified means. Reduced to a gaseous state, the Gelth eventually made their way to Earth, where they were able to use a minor rift in reality to manifest in the form of blue vapour, subsequently using human corpses acquired from the funeral parlour run by Gabriel Sneed, which had been built over the rift that the Gelth were using. Fortunately, his maid, Gwyneth, possessed a degree of psychic ability due to the amount of time she had spent in the house, allowing her to track down any Gelth that escaped in their borrowed bodies before anyone could realise what was happening.
When the Ninth Doctor and Rose were visiting Cardiff in the past as Rose’s first trip into history – although The Doctor had actually been aiming for Naples in 1860 (“The Unquiet Dead”) , they witnessed one of the Gelth’s hosts being ‘recaptured’, Sneed subsequently chloroforming Rose when she tried to interfere. When The Doctor hitched a lift in a carriage to follow Rose, he only realised after he entered the carriage that it was actually owned by Charles Dickens himself (Although Dickens decided to let The Doctor remain after The Doctor complimented his work).
Tracking Sneed to the funeral house, The Doctor was able to realise what was happening, arranging a séance so that the spirits could use Gwyneth to explain what was happening. Although The Doctor suggested that the Gelth could use human corpses as a temporary measure until he could find a more permanent home for them, when Gwyneth opened herself up to the rift to act as a ‘bridge’ that the Gelth could use to enter this world, it was revealed that the number of Gelth was actually far larger than they had claimed, and they intended to kill humanity to provide them with new bodies.
Although initially shaken at what he was witnessing, Dickens was able to come up with a new plan while the Gelth attempted to attack The Doctor and Rose after they locked themselves behind another door in the basement. Realising that the vaporous Gelths were affected by gas, Dickens turned off the gaslights in the funeral parlour while turning the gas up, drawing the Gelth out of their bodies. With Gwyneth essentially already dead after she became the bridge, the remaining fragments of her consciousness sacrificed herself to seal the rift by striking a match, setting the gas on fire and destroying the parlour, the rift-bridge, and the Gelth that had passed through it already.
With Gwyneth’s sacrifice, the Gelth’s means of accessing reality was sealed, although her death did create the Cardiff Rift, a scar in time and space that could allow various alien beings access to Cardiff. This rift was subsequently monitored by the ‘Torchwood’ organisation – the Cardiff branch that controlled the rift eventually coming under the direct control of The Doctor’s companion Captain Jack Harkness, and including among its members Gwen Cooper, a distant relative of Gwyneth , as well as being occasionally used by the TARDIS as fuel (“Boom Town” and “Utopia”), until it was finally sealed when the Eleventh Doctor triggered a new Big Bang to restore the universe after the TARDIS nearly caused a total event collapse (“The Pandorica Opens/The Big Bang”).