Rigorous studies of the impact of digital games on student learning remain relatively rare, as do studies of games as supports for learning difficult, core curricular concepts in the context of normal classroom practices. This study uses a blocked, cluster randomized controlled trial design to test the impact of a digital game, played as homework prior to instruction, and associated supplemental instructional activities, on middle grade students' understanding of the process of photosynthesis. The role of the teacher as a potential moderator of the game's impact on student outcomes was also investigated, using Classroom Assessment Scoring System-Secondary Edition (CLASS-S) observations as a measure of instructional quality. Study findings demonstrate that the intervention did not have a significant impact on student understanding of photosynthesis. The interaction of treatment teachers' CLASS-S scores and students' average photosynthesis assessment scores approached significance. This study suggests that when digital games are used as a step in the process of learning difficult conceptual material, teachers may need support and guidance to make productive connections between in-game experiences and the target concepts.