In inquiry-based science lessons teachers face the challenge of adhering to curricular goals while simultaneously following students' intuitive understandings. Improvisation (improv) provides a useful frame for understanding teaching in these inquiry-based contexts. This paper builds from prior work that uses improv as a metaphor for teaching to present a translated model for analysis of teaching in an inquiry-based, elementary school science lesson context. We call our model "instructional improv," which shows how a teacher spontaneously synthesizes rules of improv with teaching practices to support student learning, engagement, and agency. We illustrate instructional improv through case study analysis of video recorded classroom interactions with one teacher and 26 first and second grade students learning about the complex system of honey bee pollination in a mixed reality environment. Our model includes the following defining features to describe how teaching happens in this context: the teacher (1) "tells a story"; (2) "reframes mistakes as opportunities"; (3) "agrees"; (4) "yes ands"; (5) "makes statements (or asks questions that elicit statements)"; and (6) "puts the needs of the classroom ensemble over individuals." Overall, we show how instructional improv helps explain how teachers can support science discourse and collective storytelling as a teacher (a) shifts power and agency to students; (b) balances learning and agency; and (c) makes purposeful instructional decisions. Findings have immediate implications for researchers analyzing interactions in inquiry-based learning environments and potential future implications for teachers to support inquiry learning.