The evolution of the plasmasphere during the moderate 22--23 April 2001 storm is studied by comparing observed plasmasphere behavior to the predictions of a simulation. The plasmasphere observations include global images from IMAGE EUV and density and flow measurements from LANL MPA. The subjective uncertainty in the EUV plasmapause was found to be 0.2L. Inward plasmapause motion was correlated with southward interplanetary magnetic field (IMF), with a 27 min delay. The electric (E) field at the plasmapause was approximately 13% of the solar wind E-field. The observations support the idea that plasmaspheric erosion begins with a partial plasmapause indentation which then gradually widens in magnetic local time (MLT) to encompass the entire nightside. In situ measurements confirm a dayside plume of sunward flowing plasma, and images show that the plume subsequently experienced phases of MLT narrowing and rotation/wrapping. To simulate the event, we employed a test particle representation of the plasmapause using a parametric E-field model that includes corotation and convection from two sources: dayside magnetopause reconnection (DMR) and the subauroral polarization stream (SAPS). The model captures the phases of plume evolution, and it reproduces the observed plasmapause with maximum (mean) error <0.7L (0.24L). The model fails to reproduce important subglobal features (e.g., a predawn plasmaspheric shoulder) and grossly misplaces the inner edge of the rotating and wrapping plume. SAPS contributes significantly to the duskside flows, but the simulated flows are about 50% smaller than those seen by LANL MPA. The simulated plasmapause E-field is about 70% of the EUV-inferred value.