A theoretical model of a mechanism of plastic deformation in high-temperature ceramic materials containing amorphous intercrystalline layers (AILs) is suggested. Within the model, the plastic deformation is realized due to the nucleation and the development of inclusions of a liquid-like phase in the AILs and the glide of lattice dislocations emitted from the triple junctions of the AILs that contain the liquid-like phase inclusions. In the exemplary case of high-temperature α-Al2O3 ceramics with AILs, the temperature dependences of the critical stresses for the formation of a liquid-like phase nucleus, for the lattice dislocation emission and for the lattice dislocation glide in a wide range of the deformation temperatures from 300 to 1500 K are calculated. The critical values of the external shear stress and the deformation temperature, at which the glide of the emitted lattice dislocations in the grain interior becomes energetically favorable, are determined.