Born's rule is the only probabilistic postulate of quantum mechanics that is not derived from its unitary dynamics. The phase-consistency condition (PCC) introduced here treats a quantum state's global phase as a beable — an intrinsic, particle-associated element of reality rather than an observable. Being individually inaccessible, this phase functions as a contextual (supplementary) hidden variable that deterministically maps each state onto a single eigenstate of the corresponding observable for a given measurement context. Over an ensemble, the detection frequencies yield the Born rule and its quadratic form, with no distribution over the phase assumed at any step. Applying Hamilton's principle of stationary action to the PCC yields a dimensionless-action formulation of the Feynman path integral. Therefore, both the probabilistic and dynamical contents of quantum mechanics follow from a single-phase condition.