The energetic storm particle event of October 20, 1989 has often been cited as an example of high-energy (≳500 MeV) proton acceleration by CME-driven shocks near 1 AU. We examine high-time resolution solar wind and magnetic field data from the IMP-8 spacecraft and energetic particle data from the IMP-8 and GOES-7 spacecraft. We show that the high-energy particle population in this event is not a locally shock-accelerated population, but rather a population of particles confined to a plasma structure with depressed magnetic field and solar wind density. This structure was bounded by enhanced densities and strong magnetic fields. Energetic protons within this structure supplied the pressure needed to prevent the structure from collapsing. The intensity and evolution of this energetic storm particle event were shaped mainly by this spatial structure rather than by the CME-driven shocks.