The Unquiet Dead

The Doctor attempts to pilot the TARDIS to Naples in 1860 to show Rose the past, but misses short, ending up in Cardiff in 1869. While they land and Rose changes into more appropriate garb, there is trouble in a nearby funeral parlour run by Gabriel Sneed and his servant girl Gwyneth - the corpse of a grandmother has been taken over by a blue vapour, killing her mourning grandson Redpath, and then escapes the parlour. Gwyneth with her clairvoyance senses that the corpse, per her last desire, is going to see Charles Dickens at a nearby music hall. In the middle of his performance, the blue vapour leaves the woman, scaring the audience away and attracting the attention of the Doctor and Rose. Gabriel and Gwyneth arrive to retake the corpse as the blue vapour disappears into the pipes, and they are also forced to kidnap Rose by knocking her out with chloroform when she confronts them. Charles accuses the Doctor of ruining his performance, but after the Doctor gushes over his literary genius, and learning that an adventure is afoot, Charles gladly joins up to help.
At the funeral parlour, Rose wakes up and is borne down by the animated corpses of Redpath and his grandmother. The Doctor and Charles arrive, breaking into the parlour and rescuing Rose; the Doctor attempts to learn from the corpses why they are doing this, and determines that the parlour is built on a rift in spacetime, and the blue vapours are beings crossing through the Rift; they are able to use the corpses for a short time, but cannot sustain them. Rose talks more with Gwyneth, and her clairvoyance is discovered, an effect of living in the parlour since her parents died, according to the Doctor. Using Gwyneth as a channel, they hold a séance to directly communicate with the beings, who they learn are called "Gelth", their bodies destroyed as part of the Time War. The Doctor offers the Gelth temporary use of corpses only until he can transport them to a place where they can build new bodies, using Gwyneth as a bridge to cross the Rift.
As the process starts, the number of Gelth is much higher than anticipated, and their true motive is revealed: they are willing to kill the living to give themselves more hosts and take over the planet. Gabriel's neck is broken and his body taken over by a Gelth as Charles flees the parlour, leaving Rose and the Doctor trapped. Outside, Charles notes that the beings are affected by gas, and returns to the house, extinguishing the gaslights and turning the gas on full. The Gelth are forced to abandon the corpses and though the Doctor tries to encourage Gwyneth to send them back across the Rift, she cannot, nor can she leave; instead, she takes out a box of matches, intending to ignite the gas and killing the Gelth along with herself. The Doctor, Rose, and Charles flee the parlour before it is engulfed in flames. As the Doctor and Rose head back to the TARDIS, Charles thanks them for their help and makes a commitment to patch up things with his family and finish The Mystery of Edwin Drood, though the Doctor notes later to Rose that Charles will die within the year, leaving that work unfinished, but they have made him feel more alive than he ever has been. The Doctor and Rose give their goodbyes and disappear in the TARDIS. An astounded and delighted Dickens walks away through the streets of Cardiff, greeting everyone he passes and quoting from his book A Christmas Carol; "God bless us, everyone!"