Metallic systems with delocalized valence electrons host collective charge density oscillations known as plasmons. On the other hand, conventional insulators do not have free electrons and the low energy charge degrees of freedom are pinned to the ions. The fate of the collective charge excitations in the intermediate regime is an outstanding question. This problem is especially important for strongly correlated systems such as the layered cuprates, where unconventional superconductivity and other emergent phenomena arise from valence electrons on the border between Mott localization and itinerancy. Using resonant inelastic X-ray scattering, we track this evolution in the prototypical electron-doped cuprate Nd2−xCexCuO4. We find a continuous transformation of the low-energy charge response: from an acoustic plasmon in the metallic regime, to a gapped mode with a dispersion anomaly at intermediate doping, and finally to a nearly dispersionless 139 meV excitation at half filling. The 139 meV excitation at zero doping has approximately twice the energy of the oxygen phonons responsible for the dispersion anomaly observed in angle-resolved photoemission spectroscopy. To explain this evolution, we introduce an inhomogeneous microscopic framework where plasmons in metallic regions couple with two-phonon excitations in insulating regions. These results establish a unified picture of collective charge excitations across the phase diagram of electron-doped cuprates and reveal a missing link in the charge dynamics of carrier doped Mott insulators.