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Environment is thought to play a central role in quenching low-mass dwarf galaxies. However comma an increasing number of dwarf galaxies discovered in isolation suggest that mechanisms other than environment can quench dwarf galaxies. Here comma we propose deep WFC3-F395N (Ca H and K) imaging to measure metallicites (virgul0.1-0.2 dex) for hundreds of individual stars in Cetus and Tucana comma the two nearest examples of isolated quenched dwarf galaxies. Our Ca H and K photometry will provide an order-of-magnitude increase in the number of stars in each systems with reliable metallicities (from virgul40 to 400) comma enabling the first detailed study of the metallicity distribution functions (MDFs) comma and the physics they encode comma in isolated quenched dwarf galaxies. Specifically comma we will (1) construct and characterize detailed MDFs for Cetus and Tucana from a number of stars comparable to MW satellites (2) use our MDFs to differentiate between signatures of external vs internal quenching in low-mass galaxies and (3) combine our MDFs comma HST-based SFHs comma and HST-based proper motions (and orbital histories) to complete our knowledge of these enigmatic galaxies. formation histories. This program will provide qualitatively new insight into how and why isolated low-mass galaxies stop forming stars. The construction of MDFs in such distant systems is only possible due to the excellent blue-sensitivity and angular resolution of HST -- no other ground- or space-based facility can acquire this data. This novel use of HST will establish it as a leader for measuring stellar metallicities in galaxies located at large distances from the Milky Way.