Dark patterns are malicious user interface design strategies that nudge users toward decisions that may be against their best interests. As countermeasures, related work has explored legislation, detecting and modifying web content with such patterns, and educating users about dark patterns. Serious games that teach detecting and classifying dark patterns, however, have received little attention so far. We present a web-based prototype of such a game targeting adults and the results of a comprehensive user study investigating its effectiveness and knowledge retention among participants. Our findings demonstrate that participants became better at detecting dark patterns, indicating that they became more aware of such design manipulations. While our analysis of participants’ classification abilities yields mixed results, our game log data confirms that their detection skills increased.