The study contributes to language teacher education research by emphasizing an English as a second language (ESL) teacher's learning through critical activity, which includes my attempts to change my pedagogical practices to provide more equitable educational opportunities for ESL students in the mainstream content classroom. Framed by Engeström's (2001) Cultural Historical Activity Theory (CHAT), the study's purpose was to examine my own professional self-development as an ESL teacher in learning how to initiate and sustain collaboration with a seventh-grade social studies teacher at a suburban middle school in the southeastern United States. Data collection included self-study methods, specifically interviews, collaborative planning sessions, reflective journals, field notes, and e-mail exchanges with a critical friend. The findings showed how I learned to navigate the misuse of planning time and the misrepresentation of collaborative teaching notions as the two major contradictions in collaboration to plan for and teach ESL students. By resolving the contradictions, I negotiated a division of labour and enacted my agency to assume the position of a content social studies teacher, which ultimately sustained the collaborative activity. These findings attend to the complex factors that influence an ESL teacher's professional self-development and agency in collaboration with a social studies teacher.