We propose a model that describes both direct and inverse Hall-Petch dependences observed in nanocrystalline ceramics as well as the low strain rate sensitivity of such ceramics. Within the model, plastic deformation in nanocrystalline ceramics is realized via the emission of lattice and grain boundary (GB) dislocations from GB steps and triple junctions of GBs. The model assumes that in the beginning of plastic deformation, the applied load linearly scales with plastic strain and that each GB step or triple junction can emit a dislocation no more than once. The model predicts that the transition from the direct to the inverse Hall-Petch dependence is associated with an increase in the number density of triple junctions as grain size decreases. It is demonstrated that the critical grain size for this transition depends on the fraction of triple junctions that can emit lattice or GB dislocation at a given stress. In turn, the intensity of GB dislocation emission from triple junctions can depend on the structure and energy of GBs and their chemical composition. The model explains the experimental observations (D. N. F. Mucho et al.,. Mater. Lett. 186, 298 (2017); C. Yang et al., J. Amer. Cer. Soc. 102, 6904 (2019)) of direct Hall-Petch dependences down to very small grain sizes by assuming that the critical grain size for the transition from the direct to the inverse Hall-Petch dependence for the materials synthesized in these experiments is smaller than the minimum grain size of the fabricated specimens.