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 any more deaths from this violent form of sex.

Ianto Jones has been hiding a very big secret at Torchwood - his half-converted girlfriend from the Canary Wharf Battle - down in the basement. He is quick to hire a Japanese doctor who believes he can "cure" her. But things soon go wrong, and the Cyberwoman's destructive powers are released to the whole of Torchwood. Upgrading is compulsory.