Plot summary
Gwen is called into work at Torchwood early when a meteor falls outside of Cardiff, cutting her date with Rhys short. As the Army secures the area, Torchwood investigate the meteor. Gwen's colleague Owen taunts her by calling her the "new girl". In an attempt to retaliate, she throws a chisel at him, but misses and cracks open the meteor, allowing a purple gaseous entity to escape. The gas finds a young woman, Carys Fletcher, outside a nightclub and takes her host. In the club, a possessed Carys seduces a man and takes him to a restroom, where they proceed to have sex. At the moment of climax, however, the man dissolves into dust while Carys absorbs the energy that remains.
Torchwood become aware of the bizarre death and realises through CCTV that the gas has taken over Carys. They later find where Carys lives and arrive there before she could harm a postman. When she tries to escape, Owen traps her using a portable prison cell. Carys is placed in a holding cell in Torchwood's hub. As tests are run on Carys, Gwen feels uncontrollable urges to kiss her, showing that Carys is emitting high levels of pheromones, effectively becoming a "walking aphrodisiac ", but the gas is also slowly killing her. Later, Carys escapes from her cell after seducing Owen.
In an attempt to capture her again, Torchwood run on Toshiko 's hunch that she will be after her ex-boyfriend Eddie; they arrive at his house to find Carys already killed him. Later they discover Carys works as a temp at a fertility clinic. They race to the clinic to find Carys has been inside for some time killing some of the clients. After eventually cornering Carys, they find that she has moments before she dies. Jack buys her some time by kissing her, transferring some of his "excess" of life onto her.
Gwen offers the gas presence her own body as a host to save Carys. When the gas leaves Carys, Jack throws the portable prison cell at it. Since it cannot survive in Earth's atmosphere for long, the gas dies out. After Carys is safely returned home, Jack advises Gwen not to let the job consume her because her perspective is important to the team. He then encourages Gwen to go home and spend time with Rhys.
Credits
Adapted from Wikipedia contributors, Day One (Torchwood). Narrative and paragraph formatting adjusted; illustrations, reference markers and non-story material omitted.