We present a description of the northern boundary of the geomagnetic tail plasma sheet as observed with the medium-energy particle experiment (NOAA/WAPS) on the spacecraft Isee 1. The energy coverage extends from 24 to 2081 keV for energetic ions and from 22.5 to 1200 keV for electrons, and essentially all spatial directions are sampled at these energies. Large-scale motions of the plasma sheet energetic particle population were observed as an apparent result of a series of magnetospheric disturbances (substorms) that took place prior to and during April 24, 1978. The plasma sheet underwent substantial contractions and expansions (thinning and thickening) observed both in the energetic particle populations and in the cross-tail magnetic field components. The three-dimensional particle observations reveal a distinct plasma sheet boundary layer where the ion and electron characteristics are very different and where the ion flow is strong, with the flow velocity essentially coinciding with the kinetic velocity. During 0000--0700 UT, Isee-1 crossed this boundary layer 5 times, and in each case the flow was seen from the tail towards the earth. The thickness of this flow layer is estimated to be less than 2 ion gyroradii for ions in our energy range. For one of the crossings the motion of the boundary layer can be interpreted as large-amplitude boundary waves superimposed on the general plasma sheet behavior associated with the substorm process. These waves have periodicities of a few minutes.