Imagine a world where dinosaurs still walk the earth. A world where the Germans won World War II and you are president of the United States. Imagine a world where the laws of physics no longer apply and where infinite copies of you are playing out every storyline of your life. It sounds like a plot stolen straight from Hollywood, but far from it. This is the multiverse. Until very recently the whole idea of the multiverse was dismissed as a fantasy, but now this strangest of ideas is at the cutting edge of science. And for a growing number of scientists, the multiverse is the only way we will ever truly make sense of the world we are in. Horizon asks the question: Do multiple universes exist? And if so, which one are we actually in?