During 124 high-speed-stream-driven storms from two solar cycles, a multispacecraft average of the 1.1–1.5 MeV electron flux measured at geosynchronous orbit is examined to study global dropouts of the flux. Solar wind and magnetospheric measurements are analyzed with a superposed epoch technique, with the superpositions triggered by storm-convection onset, by onset of the relativistic-electron dropouts, and by recovery of the dropouts. It is found that the onset of dropout occurs after the passage of the IMF sector reversal prior to the passage of the corotating interaction region (CIR) stream interface. The recovery from dropout commences during the passage of the compressed fast wind. Relativistic-electron-dropout onset is temporally associated with the onset of the superdense ion and electron plasma sheet, with the onset of the extra-hot ion and electron plasma sheet and with the formation of the plasmaspheric drainage plume. Dropout recovery is associated with the termination of the superdense plasma sheet and with a decay of the plasmaspheric drainage plume. When there is appreciable spatial overlap of the superdense ion plasma sheet with the drainage plume, dropouts occur, and when that overlap ends, dropouts recover. This points to pitch-angle scattering by electromagnetic ion-cyclotron (EMIC) waves as the primary cause of the relativistic-electron dropouts, with the waves residing in the lumpy drainage plumes driven by the superdense ion plasma sheet. The drainage plume is caused by enhanced magnetospheric convection associated with southward (GSM) magnetic field after the IMF sector reversal. The superdense plasma sheet has its origin in the compressed slow wind of the CIR.