The Invasion - S6-E3
Trivia: In the animated reconstruction, released in 2006, the words "Bad Wolf" can be seen on a wall at one point. The phrase was a major plot point in the first season of the new series, where it turned out in the season finale, "The Parting of the Ways", that companion Rose Tyler had scattered them across time and space as a message to herself while temporarily possessing the power of the entire Time Vortex.