This study explored a middle school science teacher's curricular sensemaking in interaction with their use of an educative storyline curriculum, aligned to the Next Generation Science Standards, that was intentionally designed for more opportunities for students' scientific sensemaking. Using a phenomenological case study methodology, we examined how the focal teacher perceived and resolved (un)certainties in their understanding that emerged in their interaction with the curriculum (i.e., their curricular sensemaking), and how the teachers' curricular sensemaking impacted opportunities for students' scientific sensemaking. Findings served to expand our notions of how teachers engage in curricular sensemaking, when this sensemaking occurs, and what teachers make sense of as they experience a new, reform-oriented curriculum for the first time. In particular, the focal teacher found two types of (un)certainty to be salient at various time points during his curriculum use: (un)certainty around students' scientific sensemaking through particular science practices and (un)certainty about how to navigate the storyline curriculum using students' ideas to drive learning forward. Cutting across these types of (un)certainty, our findings suggest the need to (1) support teachers in problematizing their own understandings about particular science practices and the extent to which their previous instruction aligns with reform-oriented conceptualizations of those practices, (2) build teachers' capacity to differentiate between various causes of student discomfort and uncertainty so that students' needs and scientific sensemaking goals can be attended to simultaneously, and (3) provide teachers with opportunities to consider how they might use a variety of participation structures to move learning forward while preserving students' rights and responsibilities for the scientific sensemaking.