This paper describes a narrative-based design framework that organizes three narrative elements (i.e., storyline, character, and quest) to support a contextual game-based environment for language learning. Using this design framework, a PlanetAdventure system was developed to examine its feasibility and its effects on student language learning. A case study with 61 college students who used the game-based system to learn English vocabulary words was conducted. The findings revealed that their learning achievement was enhanced after using the system. The majority of the students favored the game-based learning experience. Based on these data and behavior logs, the students can be sorted into three clusters with different learning patterns. The findings and implications of developing contextual game-based learning environments are also discussed.