We investigate an unusual sharp boundary separating two plasma populations (inner magnetospheric plasma with high fluxes of energetic particles and plasma sheet) observed by the Cluster quartet near its perigee on 16 December 2003. Cluster was in a pearl-on-string configuration at 05:00 MLT and mapped along magnetic field lines to ~8–9 R_E in the equatorial plane. It was conjugate to the MIRACLE network and the DMSP F16 spacecraft passed close to Cluster footpoint. The properties of the sharp boundary, repeatedly crossed 7 times by five spacecraft during ~10 min, are: (1) upward FAC sheet at the boundary with ~30 nA/m² current density at Cluster and ~2000 nA/m² at DMSP; (2) the boundary had an embedded layered structure with different thickness scales, the electron population transition was at ~20 km scale at Cluster (<7 km at DMSP), proton population had a scale ~100 km, while the FAC sheet thickness was estimated to be ~500 km at Cluster (~100 km at DMSP); (3) the boundary propagated in the earthward-eastward direction at ~8 km/s in situ (equatorward-eastward ~0.8 km/s in ionosphere), and then decelerated and/or stopped. We discuss the boundary formation by the collision of two different plasmas which may include dynamical three-dimensional field-aligned current loops.