Making use of favourable local circumstances an interdisciplinary team of content-experts, educational researchers, game-designers and secondary school teachers was formed. Using a design-research approach, the team simultaneously developed and tested a serious game for the teaching off electrical circuits and effective classroom practice using it. The project comprised two rounds. In a first round, an open-inquiry teaching strategy was used and the game was observed to have a strong impact on students understanding, though not in the desired way. On the basis of this evaluation and additional expert-review, the game was redesigned. Simultaneously classroom practice was improved accordingly. An extend teacher guide was developed, supporting teachers in switching between 'free' gaming episodes in classroom and episodes of discussion and reflection. In the second round a strong impact on students' conceptual understanding of electrical circuits was observed and the students showed fewer misconceptions. Moreover students spontaneously reported that the game had helped them to understand the subject. We conclude that multi-disciplinaire collaboration was essential for this result, and that teachers played a critically role in connecting the development of the game and classroom practice. The design research approach taken greatly helped to keep the teams focus on the educational output. Accordingly, the educational expert appeared to be best positioned to play a leading role. It was also concluded that when a suitable mental model is coherently represented in the game's structure and layout (looks included), a serious game may significantly contribute to students' conceptual understanding.