Doctor Who's lengthy first run ended in 1989, and while an attempt was made to resuscitate the series in 1996, it wasn't until 2005 that it finally returned under the leadership of executive producer Russell T. Davies. The first season of the revival focused mainly on introducing the ninth iteration of the Doctor (Christopher Eccleston) and his familiar foes like the Daleks, but also added fan favorite companion Rose (Billie Piper)—and a new season-long mystery.
Rose's adventures on the TARDIS took her to many far-flung locations, but everywhere she and the Ninth Doctor went, the words "Bad Wolf" always seemed to follow. Boyfriend Mickey (Noel Clarke) owned a video game with the same name. A little boy painted the words on the side of the TARDIS in the episode "Aliens of London." There's the Bad Wolf Corporation in "Bad Wolf" and Bad Wolf TV in "The Long Game." Finally, in the season one finale "The Parting of the Ways," it's revealed that the "Bad Wolf" is Rose herself. Having looked into the heart of the TARDIS, she absorbed its energy, destroyed the Daleks, and scattered the words "Bad Wolf" throughout the universe as clues to help direct her to that moment. Unfortunately, the Ninth Doctor was forced to absorb her energy into himself to save her, causing him to regenerate into the Tenth Doctor (David Tennant).
Other subtle "Bad Wolf" references popped up during season two in episodes like "Tooth & Claw" and "Love & Monsters." During what many fans consider the most heartbreaking moment in the show's history, the Doctor and Rose say a tearful goodbye in Darlig Ulv Stranden, a.k.a. Bad Wolf Bay, in the episode "Doomsday."
The "Bad Wolf" subtly reappeared in clues throughout season four as well, as when Rose showed up for the season finale, "Journey's End." For the 50th anniversary episode "The Day of the Doctor," the Moment takes the form of Rose as the "Bad Wolf," interacting with John Hurt's War Doctor. It's a nice "moment," all puns intended, that brings the series full circle all the way back to season one of NuWho.