Recent reforms in K-12 science education call for curricular materials that are designed to be "coherent for students." This form of coherence arises when the classroom community sees their science work as addressing their questions and problems. In enactment, the teacher sometimes has to improvise from the planned trajectory of the storyline to follow students' emergent science work. In this single instrumental case study, I used interaction analysis to identify and describe episodes of whole group storyline discussion where one expert middle school science teacher worked with his students as their science work diverged from the storyline. The descriptive account for each episode documents how the teacher attended to the conceptual, epistemic, and/or interactive facets in the substance of students' talk. It also traces how collective knowledge objects (i.e., consensus model, driving questions board, and experimental graph) were developed through this attentional work in each episode. The results suggest that the teacher facilitated episodes of principled improvisation (PI) related to the students' interactive role in the discussion, the science ideas they raised for a consensus model, and the measurement errors they experienced in an investigation. These episodes were principled because the teacher foregrounded particular trajectories in the storyline to mediate and add rigor to the students' divergent lines of science work. They were improvisational because the teacher backgrounded other storyline trajectories to more effectively address and work with students' emergent ideas. In each PI, the teacher shared epistemic agency with students to co-develop each collective knowledge object. These findings show the analytical and practical value in conceptualizing epistemic agency and teachers' instructional design work in relation to students' convergent and divergent science work in the curriculum.