We assessed mind wandering (MW) during film comprehension. We predicted that prior-knowledge would aid in the construction of a situation model of the film, which would suppress MW by directing attention towards task-related thoughts, and that interest would moderate this effect. In our experiment, 108 participants either read a short story that depicted the plot (i.e., prior-knowledge condition) or read an unrelated story of equal length (control condition) prior to viewing the 32.5 minute film The Red Balloon. Participants self-reported their interest in viewing the film immediately before the film presentation and reported self-caught instances of MW while viewing the film. The prior-knowledge condition reported less MW compared to the control condition. MW also decreased over the course of the film, but only for the prior-knowledge condition, thereby suggesting a suppression effect. Finally, prior-knowledge effects on MW were only observed when interest was average or high.