Millions of dollars and uncounted hours have gone into the development of research-based instructional strategies for the improvement of undergraduate physics. However, many of these instructional strategies have extremely limited impact beyond the developer(s). Peer Instruction is the best-propagated research-based instructional strategy in undergraduate physics. We use case study methodology to construct the story behind how Peer Instruction became widely known and widely used. This case study uses interviews with the original developers and document evidence such as publications and presentations to build a rich description of the early events and propagation strategies used (knowingly or not) that have led to the spread of Peer Instruction. Peer Instruction's primary dissemination vehicle was story, through talks and the user's guide (which closely aligns with the talks). Further, many of the talks (perhaps as many as 600 in total) were given as departmental colloquia, thus leading to buy-in from departments as a whole unit. Finally, the Peer Instruction User's Manual gave potential adopters the tools they needed to implement it quickly and easily, and the team continually supports potential adopters on an individual, involved basis. These findings have implications for physics education researchers who want to increase the impact of their work.