At one time, the Gelth were a race of beings with physical forms. However, when the Time War devastated the higher forms - which apparently included the Gelth - their bodies wasted away, leaving them trapped in a gaseous state akin to blue wisps of mist. They want to live again, but the only way for them to have another body is to inhabit corpses; when a body decomposes, it breaks down and produces gas, making it the perfect vehicle for the Gelth to slip inside. Some of the Gelth are able to travel to Earth through a rift in time and space that leads to Cardiff in 1869; once there, they spend three months "haunting" a funeral home and periodically inhabiting the dead bodies. However, their journey through the rift weakens them, and they can only inhabit the dead bodies for so long. When they are unable to hold onto the bodies any longer, they revert to their gaseous form and flow out of the corpse with a shrieking wail. Because the Earth's atmosphere is hostile to them, they then hide in, and travel through, the gas pipes. (The flames seem to feed on some of their energy, but apparently do not injure them.)
The Doctor is able to communicate with the Gelth using a medium, Gwyneth, who had grown up over the rift and thus possessed certain psychic gifts. Through Gwyneth, the Gelth claim that they are trapped in the rift and dying, unable to sustain their human forms. They ask the Doctor to take Gwyneth to the rift to create a bridge, along which the Gelth can travel to inhabit the corpses of Earth. The Doctor and Gwyneth agree, on the condition that once the Gelth have all crossed over, he will take them someplace where they can build more permanent bodies.
As promised, Gwyneth opens the rift and lets the Gelth through; but they have lied. They are not few in number, but numerous, and they come through the rift in force. All of them need corpses, and they are not satisfied with merely taking the corpses that happen to be lying around; instead, they decide to make more. The Gelth-possessed corpses rise up and begin to kill people, who the Gelth immediately inhabit. They declare their intention to kill the entire human race, "converting" them into vessels for the Gelth.
Realizing that the Gelth prefer the gas pipes over untainted air, Charles Dickens comes up with a solution: turn off the gas-lamp flames and fill the room with gas, which draws the Gelth out of their hosts like poison from a wound. Although Gwyneth is unable to send them back through the rift, she is able to keep them contained in the house as they swirl around the rafters. While the others escape, she strikes a match, which ignites the gas, killing the Gelth and closing the rift.
