The (Tenth) Doctor seeks Rose Tyler after he loses her to the parallel universe. He ends up, instead, in the universe of the wizarding world, also known as the world of Harry Potter and Lord Voldemort. The TARDIS is unable to exit the alternate universe, and the Doctor is stuck, still dreadfully missing Rose, in a world where magic exists and is common. In this AU, the Doctor goes a bit insane missing Rose in the different universe, and disguises himself as Barty Crouch (JUNIOR) for a while. And ends up serving the Dark Lord. Enjoy. (All characters and settings from the show Doctor Who and the book series Harry Potter belong to BBC and Steven Moffat, and J.K. Rowling.)