A model of a 3D classical particle built up of micro-scale spinor fragments, hence structured, is proved sufficient (demands no additional postulates) to generate a theory of relativistic motion, provided the velocity of light in vacuum is the speed limit. In this case, the particle’s structure endows a mechanical action with a clear geometric sense and allows for derivation of a Lagrangian function of the same form as in special relativity. But the graphic image of motion becomes a 3D helix instead of a 4D space-time world line. A forced distortion of the helix line (curved and compressed) results in emerging of variable metric components thus leading to an equation of particle motion under the influence of a geometric potential, an equation identical to that of 4D geodesic. The logical line of the theory is traced in detail.