This study presents a comprehensive three-dimensional (3D) characterization of lath martensite and its internal boundary networks in a low-carbon 13Cr-4Ni stainless steel (CA6NM cast alloy), utilizing large-volume Xe+ plasma focused ion beam (PFIB) serial sectioning and electron backscatter diffraction (EBSD). Based on the 3D EBSD data, the martensitic microstructure was segmented into prior austenite grains (PAGs), packets, blocks, and sub-blocks, enabling statistical analysis of these structural units. The 3D networks of sub-block, block, packet, and PAG boundaries were identified, quantified, and classified using the Kurdjumov–Sachs (K-S) orientation relationship and its associated intervariant misorientations, providing new insights into the morphology, crystallography, and spatial distribution of internal boundaries. The dominant intervariant boundaries were found to predominantly terminate on {110} planes, exhibiting symmetric tilt, twist, or mixed character depending on the misorientation axis, underscoring the anisotropic crystallographic nature of the martensitic boundary network. In addition, boundaries with a 60°/[011] misorientation exhibited the highest degree of connectivity and continuity across the 3D microstructure. Morphological analysis further revealed three primary types of interactions between martensitic features: hard impingement of blocks from different packets, mutual intersection of blocks from distinct packets, and interpenetration of sub-blocks or blocks within a single packet. These interactions contribute to the formation of an interlocked martensitic microstructure, characterized by inhomogeneous boundary networks with complex morphological and crystallographic features. These new insights highlight the advantages of advanced 3D techniques in capturing microstructural intricacies, offering a robust foundation for developing predictive models that link microstructure to the mechanical performance of these alloys.