Interaction-free measurement, counterfactual communication, and Young’s double-slit experiment reveal how quantum systems exhibit effects from events that do not physically occur. In this paper, we reinterpret such phenomena through the Hidden Deterministic Interpretation (HDI), which introduces a physically fixed global phase as a hidden variable. HDI employs two principles—the Phase Consistency Criterion (PCC) and the Quantum Hamilton’s Principle (QHP)—to determine the particle’s definite trajectory within a physically real wavefunction of Heisenberg's picture. Even paths not taken influence outcomes through interference, as seen in the double-slit experiment. Detection ends the particle's trajectory, naturally rendering the prior wavefunction obsolete without invoking stochastic collapse or many-worlds branching. HDI thus provides a unified, deterministic account of quantum counterfactuality within a realist, single-world framework.