Mission HydroSci is a game-based 3D virtual environment for enacting transformational role-playing for middle school science students. Student-players will be engaged in a narrative about needing to investigate water resources and use scientific argumentation to complete missions critical to the survival and accomplishments of the members of their scientific enterprise. Our poster presents our progress in years 1 and 2 of a funded project to integrate pedagogical and gameplay objectives, and build mechanisms for purposeful player engagement and activity. The paper describes the argumentation engine as an example of developing a mechanism to enable learning. In additions, several innovative efforts have been made to support learning argumentation processes in games. MHS is being developed through a set of evaluations that support decision-making about design and development choices and eventually lead to a field test to assess outcomes. [This paper was published in: "Proceedings of the 12th International Conference on the Foundations of Digital Games" (Article 47). New York, NY: ACM.]