Education and training are among the fields taking advantage of serious games (SGs). In this article, we present Gea 2: A New Earth, a digital SG developed as an immersive 3-D virtual learning environment, integrating several educational resources, including multimedia learning material, communication tools, and intelligent tutoring support. The game aims to complement traditional classroom activities in science, technology, engineering, and mathematics (STEM), for high school students and teachers. It incorporates an intelligent pedagogical agent that can converse in natural language with the student and provides unsolicited hints during gameplay. This article presents the game and its evaluation based on experiments involving about 100 participants. We think that the results presented here add to the research on game-based learning for STEM, by proposing a complex game system, where artificial intelligence techniques are integrated to support students' learning, and by confirming that the game experience can be attractive for the learners, also in very constrained classroom environments, such as those we operated in.