Why do people endorse conspiracy theories? There is no single explanation: different people have different attitudes to the theories they say they believe. In this paper, I argue that for many, conspiracy theories are serious play. They're attracted to conspiracy theories because these theories are engaging: it's fun to entertain them (witness the enormous number of conspiracy narratives in film and TV). Just as the person who watches a conspiratorial film suspends disbelief for its duration, so many conspiracy theorists do not believe the theories they endorse; rather, they suspend disbelief in them. I argue that the serious play hypothesis explains some characteristic features of conspiracy theories, such as their gamification and the kind of relationship they have to evidence.