The Doctor (Christopher Eccleston) and Rose (Billie Piper) land in Cardiff, Wales, on Christmas Eve of 1869. At a funeral parlour run by Gabriel Sneed (Alan David) and his servant Gwyneth (Eve Myles), the body of Mrs. Peace (Jennifer Hill) has been inhabited by a mysterious blue vapour. She kills her grandson Mr. Redpath (Huw Hayes) and escapes. Gwyneth is clairvoyant and knows that the corpse is on its way to see Charles Dickens (Simon Callow) at a theatre. In the middle of his performance, the blue vapour emerges from Mrs. Peace’s body and scares the audience away. The commotion attracts the Doctor and Rose. Sneed and Gwyneth arrive and seize the walking corpse but are confronted by Rose and seize her too.

She wakes up at the funeral parlour with the corpses of Mrs. Peace and Mr. Redpath. The Doctor and Dickens arrive, break in, and rescue Rose. The Doctor talks Gwyneth into letting him hold a séance to communicate with the corpses. The blue vapours turn out to be the Gelth, who were killed in the Time War and are now living in the gaslight. There is a Time Rift below the morgue, and they beg the Doctor to open it and let them all cross over. The Doctor offers to transport them to a place where they can build new bodies, using Gwyneth’s psychic powers to cross the rift.

She does that, but there are billions more Gelth than expected and it becomes plain that they intend to kill the living people of Earth and take over the planet in their bodies. Sneed is possessed. Dickens extinguishes the gaslights and turns the gas on full, pulling the invading Gelth out of their bodies. The doctor tells Gwyneth to send the Gelth back and close the rift. But she takes out matches, intending to ignite the gas and destroy them with the explosion. The Doctor, Rose, and Dickens escape before the building explodes. Dickens vows to write about what he has seen in The Mystery of Edwin Drood, but the Doctor tells Rose that Dickens will not live to finish the book. The author watches in astonishment as the TARDIS fades away.

After an introductory episode in the present time, and one in the future, this episode was intended to show the range of the series by travelling to the past. It introduced the Cardiff Rift, which was to feature prominently in the spinoff series Torchwood, launched the next year. Eve Myles would play Gwen Cooper in that series because she impressed everyone with her performance as Gwyneth. The writer, Mark Gatiss, was a Dickens fan. Actor Simon Callow, famous for playing Dickens in his one-man show, at first balked at playing the role because Dickens was usually just presented as a Victorian cliché, but Mark Gatiss won him over with his knowledge of the man.

Gwyneth looks into Rose’s mind and sees the Big Bad Wolf, which becomes important in later episodes. David Tennant was originally supposed to play a younger Gabriel Sneed but was hired to play the Doctor when Christopher Eccleston bowed out. Because Cardiff didn’t have enough Victorian architecture, the story was mostly filmed in Swansea, Monmouth, and Penarth. Though one reviewer groused that the turncoat Gelth suggested that asylum-seekers were dangerous (Some people think those who believe everything is political are dangerous), most enjoyed the chilling tale and the Victorian interiors. Others thought that the Doctor and Rose had not much to do in the story and were little more than spectators. When in doubt, or when running out of money, Doctor Who always turned to period drama, which the BBC could do with its eyes closed.