Via a systematic review of the literature on learning games, this article presents a systematic discussion on the design of intrinsic integration of domain-specific learning in game mechanics and game world design. A total of 69 articles ultimately met the inclusion criteria and were coded for the literature synthesis. Exemplary learning games cited in the articles reviewed and developed by credible institutions were also analyzed. The cumulative findings and propositions of the game-based learning-play integration have been extracted and synthesized into five salient themes to clarify "what," "how," "where," and "when" learning and content are embedded in and activated by gameplay. These themes highlight: (a) the types of game-based learning action--prior-knowledge activation and novel-knowledge acquisition, (b) the modes in which learning actions are integrated in game actions--representation, simulation, and contextualization, (c) the blended learning spaces contrived by game mechanics and the game world, (d) the occurrence of meta-reflective and iterative learning moments during game play, and (e) the multifaceted in-game learning support (or scaffolding). Future directions for the design and research of learning integration in digital games are then proposed.