Looking closely at lesson plans as mediating tools, this study examines preservice teacher learning. By using activity theory in our analysis of lesson plans and other data collected in a student-teaching course, we uncovered tensions within/across the contexts of university teacher education program and secondary school field placements. This study serves to further understandings of how new teachers learn to appropriate the genres of teaching and to explore contradictions between a university preparation program and secondary school contexts. Findings provide an account of the student--teachers' lesson planning and an explanation of how disruptions in those plans provided insight into points of tension. By viewing these tensions as contradictions within/across activity systems, we identify them as opportunities to better understand teacher learning and ways we might support new teachers in navigating tensions in their classrooms and schools.