The paper presents a design case study of BIG GAME, a digitally-mediated seminar game aimed at developing environmental literacy and STEM skills among middle-school students. The game employs a series of open-ended missions based on real-life environmental issues, framing the missions as wicked problems, i.e., complex problems that resist straightforward solutions due to their evolving and often contradictory requirements. This approach was chosen since real-life environmental issues fall under the domain of wicked problems, yet traditional education largely focuses on unambiguous problems with clear-cut solutions. The paper introduces the choices taken during the design process and reflects on the results of implementing the game as a competition involving 27 schools in four European countries.