In this study, we analyse 23 prospective teachers' lesson plans for short whole-class mathematics discussions held with students at a middle school. The plans are in the form of "lesson plays", where an imagined discussion between the teacher and students is written verbatim. We use the Knowledge Quartet as a framework to investigate what insights about prospective teachers' professional knowledge for mathematics teaching lesson plays can reveal. Our study shows that lesson plays provide thick information about the prospective teachers' ability to transform and connect mathematics in order to make it accessible for students. One lesson play and its analysis are presented in their entirety to give insights into the data and our use of contributory codes from the Knowledge Quartet in the analysis. Implications of the study for teacher education are considered in the conclusion. [Note: The volume number (1) shown on the PDF is incorrect. The correct citation is v21 n1.]