Carys Fletcher was a Cardiff girl in her late teens or twenties living at home with her father, until the sex gas possessed her.
One evening during a club, after splitting up with her boyfriend, Eddie Gwynne, the sex gas entered into her body and used it as its vessel. Taken over by it and now able to generate powerful pheromones, she seduced and killed a young man at the club named Matt. The gas fed by stimulating its victims to the point of orgasm, at which stage their bodies exploded into dust.
Torchwood 3 captured her with a portable prison cell took her back to the Hub for questioning until she persuading Owen Harper of Torchwood to get her to release her. Amongst other men, she eventually killed her ex-boyfriend. After the team managed to separate the gas from Carys, she returned to her previous life.