In this paper, I report on doctoral research in which I studied the changes in mathematical knowledge and beliefs of two Year 5/6 teachers as they implemented a four-week innovative curriculum unit. Such immersion experiences have the potential to develop teachers' understanding of mathematics in the context of the classroom. Year 6 case study teacher Debbi's experience is discussed in relation to curriculum fidelity and opportunity to learn, in particular the foregrounding of higher achieving students. Debbi's firmly entrenched practices, related beliefs, and affective response to the curriculum presented as the dominant filters for reflection and enaction.