The aim of this study was to investigate the use of an educational computer video game in teaching and learning. Cultural-historical activity theory is used heuristically to explore the social and cultural interactions during game play. It is argued that knowledge construction occurs when video games function as a tool to mediate learning rather than as instructional media. The unit of analysis is not the game as instruction but engagement with the game. Twelve 14 to 19 year old black orphans from Soweto, South Africa, participated in a case study. Groups of three participants, which included both sexes, played the game for at least six hours, kept a personal reflective journal, and after play answered a knowledge test and participated in a round-robin discussion. Results show that participants gained new knowledge, recognised that the game mediated their learning, identified the object of the activity and discussed how they might help their community. Results support the use of games as tools to mediate learning. (Contains 1 table and 6 figures.)