Primary school science teachers provide vague descriptions of students’ engagement in classroom communication. Furthermore, there may be a discrepancy between what science teachers consider important to teach and what is actually taught. The transformation of intentions into teaching often involves lesson plans which require considerable time and effort to construct. Interestingly, expert teachers tend to rely on, for example, teaching strategies, rather than elaborate plans to realize intentions. This case study contributes knowledge about the congruence between intentions, plans and teaching. A grade 6 teacher was followed during her transformation of the science syllabus into teaching, and data were gathered from interviews, documents, and observations. Content analysis and inductive thematic analysis enabled a comparison between intentions and teaching. Results suggest that the comprehensive lesson plan constituted an obstacle, rather than a tool for realizing the main intention of students practicing reasoning in science classes. The discrepancy did not stem from the teacher’s rejection of the curriculum, but rather from a lack of appropriate teaching strategies to allow for greater flexibility as she struggled to cover the curricular content. The results have the potential to be useful for engaging pre-service and in-service teachers in developing flexible teaching strategies and time efficient planning approaches in upper primary science.