The energetic particle event associated with the quasi-perpendicular interplanetary shock which passed ISEE 3 on June 6, 1979, is characterized by persistent beamlike antisunward particle fluxes on both sides of the shock. We found that the shock has no significant nonadiabatic effects on the energetic particles near 1 AU. The lack of particles with pitch angles larger than 60¿ accounts for the absence of signatures of shock drift acceleration. The adiabatic behavior of the bulk of the particles at the shock offers a unique opportunity to understand the role of the postshock magnetic regime. A recently formed magnetic discontinuity just downstream from the shock forms an effective obstacle for particles, particularly those with a small gyroradius. From the spatial dependence of the particle population in front of the magnetic discontinuity we derived the escape probability for particles to cross the discontinuity. Strong anisotropic particle bursts are observed as intensity spikes (duration less than 1 min) both upstream and downstream from the shock. Velocity dispersion in some of these spikes is consistent with impulsive release at the magnetic discontinuity. We propose that the spikes are accomplished by a disturbance propagating along the discontinuity which develops a normal component perhaps accompanied by induced electric fields, thus enabling, in particular, low-energy particles with their small gyroradii to cross and stream into the upstream region. The energetic particle fluxes in this event are among the highest we have observed at ISEE 3. These fluxes are not produced by shock acceleration near 1 AU, but originate somewhere else far downstream from the magnetic discontinuity. From what we have learned from the unambiguous particle population signatures in this event, we believe that most shock-associated particle events cannot be understood on the basis of shock interactions alone; rather, the intensity history of energetic particles below 1 MeV is determined by the complete ensemble of magnetic structures embedded in the compressed plasma well behind the shock.