The year is 2050 and an energy crisis is Earth's main concern. A brilliant scientist in Trinity City develops the Trinity energy, a new source of energy powerful enough to supply the entire world. The alien empire of the Elda learn of this energy and plot to seize it by force. They attack Earth's space colony on Jupiter's moon of Io and occupy it. Meanwhile, the Trinity energy has been applied to three giant robots that combine to form God Sigma, a mighty robot led by martial artist-turned-pilot Dantou Shiya. With its mighty sword, it now stands between the peace of Earth and the power of the Elda forces.