Understanding increased achievement as a dynamic relationship between student engagement and mathematical understanding, we designed an intervention to address both factors. Our intervention was the instructional routine number strings, led by an undergraduate tutor. In this paper, we provide a case study of the development of a fourth-grade student (Inez) who participated in our number strings intervention. Despite a complex set of initial partial understandings about multiplication and arrays, Inez demonstrated growth both in her conceptual understanding of multiplication and engagement in mathematical discussion. Shifts in engagement were related to the tutor providing physical models of arrays and the tutor checking in frequently with the student. Shifts in conceptual understanding were prompted by her scaffolded engagement in mathematical discourse in which she was asked to reflect on her mathematical understanding of the spatial structuring of arrays.