This paper summarizes the hydrogen-related crack propagation behaviour and the deformation microstructure evolution accompanying crack propagation in high-strength martensitic steel investigated by FIB-SEM serial sectioning and TEM. We found that even very small low-angle PAGB segments (sub-micrometre in size) impeded hydrogen-related intergranular crack propagation. Additionally, localized plastic deformation sufficient to produce an ultrafine-grained structure was sometimes observed at the tip of the arrested intergranular cracks. The crystal orientation changed abruptly within 1 μm of the hydrogen-related quasi-cleavage crack tip. At the hydrogen-related quasi-cleavage crack tip, a high dislocation density and deformation microstructures, including the formation of low-energy dislocation structures, were noted. However, the microstructure approximately 1.58 μm away from the crack tip was similar to that in the non-deformed state. Therefore, intense localized plastic deformation played a key role in the hydrogen-related quasi-cleavage crack propagation. The intense localized plastic deformations accompanying the arrest of intergranular cracks and the propagation of quasi-cleavage cracks contributed to intrinsic crack growth resistance in the hydrogen-related fractures.