The face-centered cubic (delta) to monoclinic (alpha) martensitic transformation in Pu-Ga is unusual because it produces a relative large change of volume up to 20%. Moreover, the transformation crystallography is not predicted satisfactorily by the classical phenomenological theories of martensite crystallography, which is based on the hypothesis that the habit plane is an invariant plane of the total shape change. In this study we use the recently developed topological model where the habit plane, i.e., the interface between the parent and martensite phases, is envisaged as a semi-coherent configuration with coherent terraces reticulated by a network of disconnections or transformation dislocations, and defects producing lattice-invariant deformation. The authors show explicitly that this network not only accommodates the coherency strains, so that no long-range strain field arises, but also produces diffusionless transformation by motion of the disconnections across the interface. It was show that the predicted habit plane inclination with respect to the parent and martensite phases in a Pu-Ga alloy is in good agreement with published experimental observations when the lattice-invariant deformation is (201)(alpha)/[10 (2) over bar](alpha) twinning, as observed experimentally.