Here we introduce Narrative Context Protocol (NCP), an open-source narrative standard designed to enable narrative interoperability, AI-driven authoring tools, real-time emergent narratives, and more. By encoding a story's structure in a "Storyform," which is a structured register of its narrative features, NCP enables narrative portability across systems as well as intent-based constraints for generative storytelling systems. We demonstrate the capabilities of NCP through a year-long experiment, during which an author used NCP and a custom authoring platform to create a playable, text-based experience based on her pre-existing novella. This experience is driven by generative AI, with unconstrained natural language input. NCP functions as a set of "guardrails" that allows the generative system to accommodate player agency while also ensuring that narrative context and coherence are maintained.