"Bad Wolf" is a recurring theme in the British science fiction television series Doctor Who. It first appears in the first series of the 2005 revival, where it serves as a mysterious message that follows the character Rose Tyler. The phrase symbolizes the influence of the Bad Wolf entity, which is a manifestation of Rose's power after absorbing the energy of the Time Vortex. Throughout the series, "Bad Wolf" is connected to various events and characters, ultimately leading to a significant climax in the finale. The phrase represents the idea of choice and consequence, as well as the impact of love and sacrifice in the face of danger.