Despite considerable interest in the development of Theory of Mind (ToM) during early childhood, until recently, there has been little consideration about whether and how ToM skills continue to change into adulthood. Furthermore, the false-belief task, which is believed to capture the underlying mechanisms of ToM, is rarely used in studies of ToM with adults; those tasks that do assess false-belief understanding may be confounded by incidental task demands, such as complex narratives and excessive memory requirements, making it difficult to isolate adults’ true ToM skills, much less to compare them with the skills of children. Here, we adapted a task developed by Valle, Massaro, Castelli, and Marchetti (2015, https://doi.org/10.5964/ejop.v11i1.829) to assess false-belief understanding in adults. Participants were randomly assigned to one of three conditions. In the reading condition, participants read a story about the unexpected transfer of a ball between three brothers. In the video condition, participants watched a video version of the same story. Finally, in the training condition, participants were first trained on the names of the characters, before watching the video. Although condition did not affect participants’ ability to correctly answer a standard false belief question (“Where does X think Y thinks the ball is?”), participants in the training condition used more mental state language to justify their responses (“Why does X think Y thinks the ball is here?”), and this improved performance was mediated by improved memory for the story details. We conclude that at least some “failures” of ToM use may be due to an inability to understand, recall, or communicate complex information in a ToM task, raising important questions about how best to measure ToM in adults (and children) in the future.