The nanoscopic interactions between glassy macromolecules and the dispersed single-walled carbon nanotubes (SWCNTs) in a nanocomposite during very large local elongations were revealed. The results also unveiled a unique mode of molecular motions followed by glassy chains during brittle nanoplastic flows. Based on detailed nanomechanical calculations on results from atomic force and electron microscopy, the molecular motions of the "strain-softened" glassy polymer chains were found highly dependent on chain entanglement density (nu(e)), in sharp contrast to the nu(e)-independent elastic reinforcement. Particularly, in the loosely entangled chains the SWCNTs behaved like "phantom tubes", manifesting no effects on strain hardening of the chain network during plastic flow. This indicates that the "glassy" chains had undergone entanglement clustering, a new and unique mode of nonaffine molecular motions. The results bear important significance in revealing the fundamental behavior of glassy polymer chains and the reinforcement by SWCNTs.