9. Silence of the Library/Forest of the Damned (Series 4, Episodes 9 & 10 - 2008)
Another core concept that Steven Moffat laid down during the Davies Era (albeit as probably more of a direct concept to reuse in his impending reign as show-runner) was that of the beautiful, yet mischievous, Professor River Song. Admittedly based in part on The Time Travelers Wife, the good Professors first appearance in The Doctors life is the last time shell ever see him. Which means that for once, The Doctor doesn't know everything. For once, he's the one that's on the other end of the Eight Ball, trying to figure out just how this stranger knows everything about everything, and how she ties into his own life's plan. As it turns out, shes destined not only to perplex him about her future role in his life, but also to sacrifice hers to protect him for their destined romance. Once again, Moffat takes the simple concept of humans and their fear of the dark and expands it into something more nightmarish and unexplained than we ever thought with the Vashta Narrada: the invisible piranhas of the shadows. Think of him as a darker, more twisted (but still equally funny) Douglas Adams.