Doctor Who

When Amy is asked by the Doctor to think of a place she has always wanted to go, she thinks she has stumped him when she says she wants to go to a fairytale place with castles and dragons. The Doctor stumps her back, and takes her to a world in the seventy-ninth century that has been made to look exactly like that, unicorns and everything. The only problem is that world's inhabitants don't know that their world isn't real.