When Torchwood investigates a meteor impact, Gwen accidentally looses an alien entity contained within, which feeds on the energies of sexual climax. The creature possesses a young woman named Carys Fletcher, who must now kill men through intercourse or perish herself. With Jack, Tosh and Owen fixated only on the alien being, it is up to Gwen to try to save Carys' life.

With executive producer Russell T Davies assuming relatively light scripting duties on Torchwood compared to his Doctor Who output, the mantle of head writer fell to Chris Chibnall, who also became the spin-off series' co-producer. A lifelong Doctor Who fan, Chibnall was the creator of the comedy-drama Born And Bred, and had also contributed to both Life On Mars and All About George.

Chibnall's first script was originally entitled “New Girl”, and was intended to be the second episode of the season. Aware that this story would feature Gwen Cooper's initial outing as a fully-fledged Torchwood member, Chibnall wanted to write the Torchwood version of the prototypical nightmarish first day on the job.

“New Girl” formed part of the programme's first production block, paired with the debut episode, Everything Changes, under the direction of Brian Kelly. It was decided that the initial phase of the recording would concentrate on “New Girl”, so that the cast interactions in the premiere would be more practised. Work on the block began on May 1st, 2006. In early October, it was revealed that the title of the episode had become Day One.

• Forthcoming.