With the landscape of gaming constantly changing, new avenues for game-based language learning are ripe for exploration. One such style of game that has gone understudied is the social deduction game, a game incorporating collaboration and competition as core, intertwined gameplay elements. This style of game, with its complex outcomes, social interaction and information exchange, has potential as a tool for language practice appropriate for intermediate learners beginning to develop complex language skills. In order to assess the viability of social deduction games, an experiment was carried out involving 5 middle and elementary school aged Japanese-speaking students playing the game Among Us using English alongside a Japanese-speaking English “coach”. Over the course of 10 weeks, the students showed increased motivation and enthusiasm, as well as an overall increase in complex language use, including use of sentences with multiple modifiers, embedded sentences and questions, and other use of language beyond a simple subject-verb-object construction. Their speech also demonstrated valuable insights into the nature of the Japanese-English interlanguage in language learners, reinforcing previous observations about the order of grammar acquisition and new evidence of the interpretation of complex structures between the languages.