Players in modern Massively Multiplayer Online Role-Playing Games progress through ambitiously designed narratives, but have no real influence on the game, since only their characters' data, not the game environment, persists. Although earlier games supported player influence by persisting changes in the world, they relied on players' capacity to form their own stories and lacked guidance for character progression. We explore how persistence and narrative emergence let us build upon players' influence rather than restrict it. We ran four studies and found that players highly value first-time and unique events, and often externalize their experiences to the Web to collaborate and socialize, but unintentionally also disrupt some aspects of in-game play. We introduce Narrative Substrates, a theoretical framework for designing game architec-tures that represent, manage, and persist traces of player activity as unique, interactive content. To illustrate and test the theory, we developed the game We Ride and deployed it as a two-phase technology probe over one year. We identify key benefits and challenges of our approach, and argue that reification of emergent narratives offers new design opportunities for creating truly interactive games.