We review the first iteration of an Alternate Reality Game (ARG) designed to engage middle school students in the interpretive process behind history and the mathematical roots of cryptography and cartography. Building from the core characteristic of ARGs as collaborative sense-making systems, we sought to integrate elements of effective cooperative learning environments, such as positive interdependence, into our game design. We detail how cooperative learning constructs were designed into the narrative and game mechanics, and how evidence of these features emerged during play. We found that the jigsaw puzzle design of game challenges supported collaboration between groups; daily, whole-class discussions strengthened group processing; and real-time, chat-based dialogue among students themselves and students with an in-game character enhanced promotive interaction (e.g., providing positive reinforcement). However, we did note weaknesses in positive interdependence within groups. We conclude with design implications for future iterations of the ARG.