15. The Impossible Astronaut/Day of the Moon (Series 6, Episodes 1 & 2 2011)
Series 5 ended with Amy and Rory (who died, came back as a mannequin, and then presumably was re-written as a man yet again) marrying and going on a honeymoon that involved A Christmas Carol, ice clouds, and flying sharks who fancied Opera. (You kinda had to be there.) After all of that, our intrepid companions deserved a bit of a rest, and a rest is exactly what they got. But while our friends enjoyed married bliss, an old friend was teasing them, calling out to them, and eventually would invite them back into the fold of adventure. It's just a shame The Doctor went ahead and died mere moments after he'd brought back his trusted friends and enigmatic love interest to join him. (Yes, River is back. Series 6 is pretty much her story arc being fleshed out.) After his death, The Doctor shows up at a diner to keep his friends together and figure out why he invited himself to his own post-death adventure. As it turns out, The Doctor HAS to die. We don't know why, or how, or WHO for that matter; but he's going to die at Lake Silencio, around 5:02 PM on April 22, 2011. With a new mystery, and new villain (The ever creepy Silence), the questions start to rise.
16. A Good Man Goes To War/Lets Kill Hitler (Series 6, Episodes 7 & 8 2011)
One such question was "Who is River Song?". Up until now, she's been showing up in The Doctor's life out of order, and she knows more about his own life than he does at times. So naturally, the only woman known to stump and confuse a nearly eternal being would draw some attention to who she is. Well, as it turns out, she's someone of great importance: she's Amy and Rory's daughter. Don't let the second part's name fool you, there is very little Hitler in the episode that bears his name. What there is a lot of, though, is timey wimey backstory involving how River became who she is and why her name mysteriously changed from "Melody Pond" to "River Song". Also, with the introduction of The Tesselecta, the mystery of who exactly River killed (something she alluded to back in "Flesh and Stone") is kick started yet again. (Here's some hints: How many "good men" are their on Doctor Who? How many can be considered "the greatest man" anyone's ever known? No, Daleks DON'T count.)