Electron density distributions and plasma dynamics in the middle- and high-latitude dayside magnetosphere are studied with observations from the IMAGE radio plasma imager. Remote measurements of the electron densities along magnetic field lines were made before and during a magnetic storm when the solar wind and interplanetary magnetic field (IMF) impinging on the magnetopause varied considerably. Several regions of different density distribution characteristics, including plasmasphere, plasma trough, subauroral/auroral density depletion, density enhancements in the aurora/cusp, and polar cap, are identified in "two-dimensional images," i.e., along the satellite orbit and field lines. The plasma dynamics, such as the plasma refilling in the outer plasmasphere and the plasma acceleration in the aurora/cusp region, are inferred from density gradients along the field lines. It is shown that the densities and locations of the plasma regions vary in accordance with the solar wind, particularly with the IMF variations for the case examined. In the partial recovery phase of the magnetic storm when the IMF was northward, the density depletion region expanded to wider latitude range and extended to lower altitudes, with much lower densities than those in the density depletion regions of the other RPI measurement periods. The density enhancements associated with the aurora/cusp region were not visible, possibly because of the diffusive nature of the dayside aurora and higher-latitude location of the cusp during this period. At the peak of the storm, characterized by a persistent southward IMF Bz, all plasma regions moved to lower L shells. The results imply that the solar wind/IMF effects should be included in any statistical study of the electron density distributions in these regions.