I argue that judgements of what is 'true in a fiction' presuppose the Reality Assumption: the assumption that everything that is (really) true is also fictionally the case, unless excluded by features of the work. By contrast with the more familiar Reality Principle, the Reality Assumption is not a rule or 'principle of generation' for inferring implied content from what is explicit in a text. Instead it provides an array of real-world truths that can be used in making such inferences. Drawing on empirical evidence, I claim that reliance on the Reality Assumption is essential to our ability to understand stories. However, the Reality Assumption has several unintuitive consequences, not least that what is fictionally the case includes countless facts that neither authors nor readers could or should ever consider. I argue that such consequences provide no reason to reject the Reality Assumption.