The current study was designed to address the following research question: Can a computer game provide an effective mechanism for training adults to identify and mitigate their cognitive biases? Human decision making relies on a variety of simple heuristic decision rules that can be quick and effective mental shortcuts when making judgments. However, these heuristics can also lead to irrational thinking and problem-solving in ways that produce errors or illogicality, known as cognitive biases. Though knowledge of cognitive biases and bias mitigation strategies can help to reduce the potential impact of cognitive biases on human reasoning, such deeply ingrained cognitive strategies are difficult to alter. The current study was designed to leverage the virtual learning environment of a serious game to take on this training challenge. To that end, a training game – Missing: The Pursuit of Terry Hughes (Missing) – was developed. Missing was created for an audience of educated adults, and the described instructional design is based on current research on effective andragogical learning theory. The Missing game design immerses the user into bias-invoking situations which provide direct experience with cognitive bias identification and mitigation strategies. In this paper, details of the game instructional design are presented, including a cognitive framework based on dual-process systems of reasoning which relates multiple biases, their causes, and mitigation techniques. An external test campaign was conducted to determine whether the game had a positive transfer of in-game experiential learning about biases to real world skills and behavior change. Results are presented that suggest this novel serious game both engages and trains players, resulting in measurable reductions in cognitive biases.