Quantum Substrate Dynamics (QSD) is a Lorentz-invariant, coherence-based field theory in which mass, gravity, and inertia emerge from phase-stable excitations within a conserved physical substrate. In this framework, mass appears as a coherence-locked phase lattice, inertia arises from reconfiguration resistance at coherence boundaries, and gravity results from large-scale substrate tension gradients. QSD reinterprets black holes, dark matter effects, and cosmological structure as coherence-driven phase transitions within the substrate field—without invoking geometric singularities or exotic matter. While it recovers General Relativity and Quantum Field Theory as limiting cases, QSD also offers falsifiable predictions beyond them, including geometry-sensitive inertia, scalar precursor waves in supernovae, and coherence-based gravitational echoes in black hole mergers. By anchoring known physics in a conserved coherence field, QSD presents a testable and physically unified extension of modern theoretical frameworks.