12. Lilith And The Carrionite Witches - The Shakespeare Code
While they may sound like a girl band from the 90s, these were the villains of a fairly important (if also fairly nonsensical) episode from Season 3. It featured Martha's first trip to the past, Shakespeare, "Expelliarmus!", Shakespeare, the first mention of Ten possibly devirginizing the Virgin Queen, Shakespeare...it was all terribly British and a good enough standalone, but the conclusion was open-ended. As the episode was more about Ten (sort of) getting over Rose and adjusting to Martha than it was about the Witches, it wouldn't be surprising if you forgot that they existed. Ten probably did too, and since he trapped the Witches and stuck them somewhere in the TARDIS, they could just be hanging out and waiting for a plot dry spell to come back. What with Moffat's constant destruction and reconstruction of the poor TARDIS, the witches could turn up any time. They just shouldn't.