"I saw the Fall of Troy! World War Five! I was pushing boxes at the Boston Tea Party! Now I'm gonna die in a dungeon... in Cardiff!"The first episode Mark Gatiss wrote for the TV series, based loosely on his Big Finish Doctor Who episode "Phantasmagoria".The Cardiff rift remains open, and becomes a major feature of the Whoniverse from this point on.
— The Doctor
The Doctor tries to take Rose to Naples, 1860. The TARDIS disagrees, and instead takes them to Cardiff, Christmas 1869.Charles Dickens is in town, prompting an outbreak of earnest fanboyishness from the usually sardonic Ninth Doctor. The squee doesn't last — the dead are walking, so the Doctor springs into action. It turns out that there's a massive space/time rift right in the middle of Cardiff. This will become important later. A young psychic maid named Gwyneth (identical relative of one Gwen Cooper) can guide things through the rift.Dragging the very skeptical Dickens along, he organises a seance and negotiates with the aliens. If he helps them through the Cardiff Rift, they can temporarily occupy some corpses until he finds them a better home. Rose is Squicked, but the Doctor isn't in the mood to listen to some silly little human and coddle her delicate sensibilities.The Gelth (as they call themselves) prove to be a bunch of lying promise-breakers, being "a few billion" in number and not being almost-dead. They were bent on wiping out humanity and taking over their corpses. The Doctor shows his raging Survivor Guilt over having previously killed a few species, most recently his own, and hesitates to jump into action. Gwyneth takes the decision out of his hands by sacrificing herself. Charles Dickens learns an important lesson about Christmas, and merrily bids the Doctor farewell, planning to include all sorts of ghosts and blue lights and aliens in police boxes in his novels from now on. The Doctor gloomily notes that nothing will actually change, though, because Dickens will be dead of natural causes within a year.