A defining symptom of autism spectrum disorder (ASD) is the presence of restricted and repetitive activities and interests, which can surface in language as a perseverative focus on idiosyncratic topics. In this paper, we use semantic similarity measures to identify such idiosyncratic topics in narratives produced by children with and without ASD. We find that neurotypical children tend to use the same words and semantic concepts when retelling the same narrative, while children with ASD, even when producing accurate retellings, use different words and concepts relative not only to neurotypical children but also to other children with ASD. Our results indicate that children with ASD not only stray from the target topic but do so in idiosyncratic ways according to their own restricted interests.