The plasma sheet is a region of hot (key) plasma extending several earth radii on either side of the neutral sheet in the geomagnetic tail. Its existence as a permanent feature of the magnetosphere has been well established observationally but has not been understood theoretically. A model is presented here that accounts qualitatively for both the origin and the dynamics of the plasma sheet. While several quantitative details remain to be worked out, the model can account for most of the observed features and behavior of the plasma sheet. The source of plasma-sheet particles in this model is the post-shock solar wind which injects particles into the magnetosphere principally through the demarcation lines on the magnetopause that separate closed (low-latitude) field lines from open (polar-cap) field lines. The particles drift longitudinally in pseudotrapped orbits and the higher-energy particles reach open field lines on the nightside and diffuse down the tail, forming the plasma sheet. The latitude of the demarcation lines determines the sharp high-latitude boundary of the plasma sheet, and the energy-dependent longitudinal drift accounts qualitatively for the energy spectrum of plasmasheet particles. The plasma-sheet particles escape downstream from the earth at a rate that is probably controlled by wave-particle scattering in the tail magnetic field, and drift into the neutral sheet at a rate controlled by the local cross-tail electric field. The plasma-sheet number density is governed by the balance between downstream loss and entry at the demarcation lines. When the loss rate exceeds the input rate, the resultant depletion of the near-earth plasma sheet may account for the enhanced rate of neutral-sheet merging responsible for the onset of magnetospheric substorms.