Six months after ridding London of the Slitheen, the Doctor takes the TARDIS to Cardiff to refuel on a rift in time and space. The Doctor, Rose and Captain Jack Harkness are joined by Mickey Smith, Rose's boyfriend. They've barely arrived when they realize that the new Lord Mayor is one of the Slitheen, Margaret, from their previous encounter. The nuclear facility she has been promoting is called Bad Wolf, an expression the Doctor has come across on several occasions. The Doctor will transport her to her home planet - where she says she faces a death warrant - but they are unable to depart until the next morning when the TARDIS is fully recharged. After unsuccessfully pleading her case to the Doctor, Margaret takes more decisive action.
—garykmcd