The Innovate to Mitigate project designed and conducted a competition for students aged 13-18 to propose and test strategies for mitigating rising levels of greenhouse gases. This paper explores the scientific inquiry and interdisciplinary learning of a 12th grade high school team over 6 months of participation. We used an activity-theoretic approach as a framework for capturing and analyzing the structure of the learning system in the team, situated within a science competition. This approach provided a lens both for finding and analyzing team learning at several levels — conceptual, procedural, metacognitive — and for revealing the processes by which learning was mediated by the activity system.