We present ENGAGE, a game-based learning environment for teaching computational thinking to middle school students. This project has dual aims: introducing computational thinking practices to students at a young age, and improving computational thinking attitudes among underrepresented students. In pursuit of these two goals, the ENGAGE team has mapped the learning objectives of the AP CS Principles course to the middle school level, and then built an immersive game experience upon that foundation. Students choose computer scientist avatars to represent themselves, and then play in pairs as they investigate a data-related mystery in an underwater research station, solving computational thinking challenges along the way. ENGAGE is currently being implemented as part of a quarterly elective in four middle schools in North Carolina. During the elective, students spend a total of ten classroom sessions playing the game, supplemented by "unplugged" activities that reinforce concepts learned in the game environment. We plan to expand to more middle schools in the 2015-2016 school year. In this demo, members of the SIGCSE community will be able to experience the ENGAGE game for themselves and learn more about its development and future directions. We will also discuss our success in recruiting and teaching the ENGAGE curriculum to middle school teachers who had no prior computer science experience, and the success of those middle school teachers in implementing ENGAGE within their classrooms.