Torchwood S01 E02 Day One

"Save the Planet (One Alien at a Time)"

It's Gwen's first day on the job, and it's not going very well. After accidentally releasing a mysterious gas from a comet that landed, a new type of alien is let loose on the streets - an alien that's addicted to sex. Torchwood must find the inhabited body, and prevent anymore deaths from the violent form of sex.