Paul Abbott was originally set to write an episode entitled "Pompeii". The story would have Rose feeling jealous of The Doctor's friendship with Jack. When they land in Pompeii in 79 AD, Jack discovers that Rose's life has been manipulated by the Doctor in an experiment to create the perfect companion. Abbot's commitment to Shameless (2004) and other projects led to him dropping out of the episode.
Russell T. Davies wanted the story to examine the moral repercussions of the Doctor's deeds - particularly in the context of capital punishment, to which Davies was personally opposed.
Russell T. Davies stated that he wanted to incorporate Welsh culture because the series is made in Wales and contains a lot of Welsh crew members. He also wanted to show off how beautiful the area could be.
The night shoot of Rose and Mickey in front of the water tower at Roald Dahl Plass had to be extended to two nights because it was below the temperature at which the fountain automatically shuts off.
Russell T. Davies wrote this episode to bring back Annette Badland as Margaret Blaine/Blon Fel Fotch Passameer-Day Slitheen due to her performance in Aliens of London (2005) and World War Three (2005) being "brilliant", though she had few lines.